Power device on bulk substrate

ABSTRACT

A metal-oxide-semiconductor field-effect transistor (MOSFET) power device includes an active region formed on a bulk semiconductor substrate, the active region having a first conductivity type formed on at least a portion of the bulk semiconductor substrate. A first terminal is formed on an upper surface of the structure and electrically connects with at least one other region having the first conductivity type formed in the active region. A buried well having a second conductivity type is formed in the active region and is coupled with a second terminal formed on the upper surface of the structure. The buried well and the active region form a clamping diode which positions a breakdown avalanche region between the buried well and the first terminal. A breakdown voltage of at least one of the power devices is a function of characteristics of the buried well.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This patent application is a continuation-in-part of and claims thebenefit of U.S. patent application Ser. No. 15/228,213 filed on Aug. 4,2016, entitled “Power Device Integration on a Common Substrate”, whichwas a continuation of and claims the benefit of U.S. patent applicationSer. No. 13/939,451 filed on Jul. 11, 2013, entitled “Power DeviceIntegration on a Common Substrate”, which was a continuation-in-part ofand claims the benefit of U.S. patent application Ser. No. 13/887,704filed on May 6, 2013, entitled “Power Device Integration on a CommonSubstrate”, which claims priority to U.S. Provisional Patent ApplicationSer. No. 61/677,660 filed on Jul. 31, 2012, entitled “Power ManagementIntegrated Circuit for Portable Electronic Devices,” the disclosure ofeach of which is incorporated herein by reference in its entirety forall purposes.

FIELD OF THE INVENTION

The present invention relates generally to electronic circuits, and moreparticularly relates to power devices.

BACKGROUND OF THE INVENTION

Modern portable electronic devices, including, but not limited to, smartphones, laptop and tablet computing devices, netbooks, etc., are batteryoperated and generally require power supply components for stabilizingthe supply voltage applied to subsystems in the devices, such as, forexample, microprocessors, graphic displays, memory chips, etc. Therequired power range is often between about 1 watt (W) and about 50 W.

Power supply/management components are usually partitioned intofunctional blocks; namely, control circuitry, driver stage and powerswitches. From the standpoint of device miniaturization, which is adesired objective of many portable electronic devices, it isadvantageous to integrate the power supply/management components into asingle integrated circuit (IC) chip. This solution is particularlydominant in very low power consumption products, where supply current islimited to a few hundreds of milliamperes (mA). FIG. 1 is a blockdiagram illustrating an exemplary power stage which includes powermanagement control circuitry 102, a driver stage 104, and power switches106 and 108, all monolithically integrated in a single IC 100.

Typically, metal-oxide-semiconductor field-effect transistor (MOSFET)devices are used to implement the power switches. A MOSFET requiresrelatively few mask steps to be manufactured (e.g., less than about tenmask levels), while control circuitry in the IC usually requires arelatively large number of mask steps (e.g., about 26 to 36 mask levels)in comparison to MOSFET devices. Consequently, an allocation of a largedie area to the power switch leads to a high product cost, which isundesirable.

SUMMARY OF THE INVENTION

Embodiments of the invention provide novel semiconductor structures andtechniques for facilitating the integration of circuits and/orcomponents (e.g., drivers and power switches) on the same siliconsubstrate as corresponding control circuitry for implementing a powercontrol device. Some embodiments of the invention provide novelsemiconductor structures and techniques for reducing gate-to-draincapacitance in a metal-oxide-semiconductor field-effect transistor(MOSFET) power device. Additionally, some embodiments of this inventionprovide novel semiconductor structures and techniques for minimizing theinjection of hot carriers into oxides of the MOSFET power device byforming a clamping diode, thus improving the long-term reliability ofthe MOSFET power device.

In accordance with an embodiment of the invention, the MOSFET powerdevice includes a bulk semiconductor substrate. An active region isformed on the bulk semiconductor substrate. A buried well having a firstconductivity type is formed in the active region on the bulksemiconductor substrate. A source region having a second conductivitytype is formed in the active region proximate to an upper surface of theactive region, the source region being electrically connected to theburied well. A drain region having the second conductivity type isformed in the active region proximate the upper surface of the activeregion and spaced laterally from the source region. A body region havingthe first conductivity type is formed in the active region between thesource region and the drain region on at least a portion of the buriedwell. A gate is formed above the active region proximate the uppersurface of the active region, the gate being electrically isolated fromthe active region by a gate insulating layer. A drain terminal is formedon the upper surface of the active region and is electrically connectedto the drain region. A source terminal is electrically connected to thesource region. A gate terminal is electrically connected to the gate,and a shielding structure is formed proximate the upper surface of theactive region between the gate and the drain region, the shieldingstructure including a field plate configured to control an electricfield distribution along a top oxide interface away from an edge of thegate nearest the drain terminal. The buried well is configured, inconjunction with the drain region, to form a clamping diode operative toposition a breakdown avalanche region between the buried well and thedrain terminal. A breakdown voltage of the MOSFET power device is afunction of one or more characteristics of the buried well.

In accordance with other embodiments of the invention, the MOSFET powerdevice includes a bulk semiconductor substrate. An epitaxial layer isformed on the bulk semiconductor substrate. An active region is formedin the epitaxial layer. A buried well having a first conductivity typeis formed in the active region in the epitaxial layer. A source regionhaving a second conductivity type is formed in the active regionproximate an upper surface of the active region, the source region beingelectrically connected to the buried well. A drain region having thesecond conductivity type is formed in the active region proximate theupper surface of the active region and spaced laterally from the sourceregion. A body region having the first conductivity type is formed inthe active region between the source region and the drain region on atleast a portion of the buried well. A gate is formed above the activeregion proximate the upper surface of the active region, the gate beingelectrically isolated from the active region by a gate insulating layer.A drain terminal is formed on the upper surface of the active region andis electrically connected to the drain region. A source terminal iselectrically connected to the source region. A gate terminal iselectrically connected to the gate, and a shielding structure is formedproximate the upper surface of the active region between the gate andthe drain region, the shielding structure including a field plateconfigured to control an electric field distribution along a top oxideinterface away from an edge of the gate nearest the drain terminal. Theburied well is configured, in conjunction with the drain region, to forma clamping diode operative to position a breakdown avalanche regionbetween the buried well and the drain terminal. A breakdown voltage ofthe MOSFET power device is a function of one or more characteristics ofthe buried well.

In accordance with yet other embodiments of the invention, the MOSFETpower device includes a bulk semiconductor substrate. An epitaxial layeris formed on the bulk semiconductor substrate. An active region isformed in and on the epitaxial layer. A buried well having a firstconductivity type is formed in the epitaxial layer. A source regionhaving a second conductivity type is formed in the active regionproximate an upper surface of the active region, the source region beingelectrically connected to the buried well. A drain region having thesecond conductivity type is formed in the active region proximate theupper surface of the active region and spaced laterally from the sourceregion. A body region having the first conductivity type is formed inthe active region between the source region and the drain region on atleast a portion of the buried well. A gate is formed above the activeregion proximate the upper surface of the active region, the gate beingelectrically isolated from the active region by a gate insulating layer.A drain terminal is formed on the upper surface of the active region andelectrically connected to the drain region. A source terminal iselectrically connected to the source region. A gate terminal iselectrically connected to the gate, and a shielding structure is formedproximate the upper surface of the active region between the gate andthe drain region, the shielding structure including a field plateconfigured to control an electric field distribution along a top oxideinterface away from an edge of the gate nearest the drain terminal. Theburied well is configured, in conjunction with the drain region, to forma clamping diode operative to position a breakdown avalanche regionbetween the buried well and the drain terminal. A breakdown voltage ofthe MOSFET power device is a function of one or more characteristics ofthe buried well.

Embodiments of the invention will become apparent from the followingdetailed description thereof, which is to be read in connection with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are presented by way of example only and withoutlimitation, wherein like reference numerals (when used) indicatecorresponding elements throughout the several views, and wherein:

FIG. 1 is a block diagram illustrating an exemplary power managementcircuit including control circuitry, a driver stage and power switchesimplemented in a single IC;

FIG. 2 is a block diagram illustrating a power stage including exemplarypower management control circuitry and a driver stage implemented in anIC, coupled with discrete power switches external to the IC;

FIG. 3 is a block diagram illustrating a power stage including exemplarypower management control circuitry implemented in a first IC, and adriver stage and power switches implemented in a second IC coupled withthe first IC, suitable for use in accordance with embodiments of theinvention;

FIGS. 4 and 5 are cross-sectional views depicting conventional laterallydiffused metal-oxide-semiconductor (LDMOS) transistor devices;

FIGS. 6 and 7 are cross-sectional views depicting conventional LDMOStransistor devices formed on SOI substrates;

FIG. 8 is a cross-sectional view depicting at least a portion of anexemplary BiCMOS structure, according to an embodiment of the invention;

FIGS. 9A and 9B are cross-sectional views depicting at least a portionof an exemplary N-channel LDMOS transistor, according to an embodimentof the invention;

FIG. 10 is a cross-sectional view depicting at least a portion of anexemplary N-channel LDMOS transistor, according to another embodiment ofthe invention;

FIG. 10A is a cross-sectional view depicting at least a portion of anexemplary N-channel LDMOS transistor, according to another embodiment ofthe invention;

FIG. 11 is a cross-sectional view depicting at least a portion of anexemplary low voltage signal MOSFET, according to an embodiment of theinvention;

FIGS. 12A to 12E are cross-sectional views depicting at least a portionof an exemplary bipolar junction transistor (BJT), according toembodiments of the invention;

FIG. 13 is a cross-sectional view depicting at least a portion of anexemplary PN diode, according to an embodiment of the invention;

FIG. 13A is a cross-sectional view depicting at least a portion ofanother embodiment of an exemplary PN diode;

FIGS. 13B and 13C are cross-sectional views depicting approaches tocoupling the gate to the anode terminal according to embodiments of a PNdiode;

FIG. 14A is a cross-sectional view depicting at least a portion of anexemplary Schottky diode, according to an embodiment of the invention;

FIG. 14B is a cross-sectional view depicting at least a portion of anexemplary Schottky diode, according to another embodiment of theinvention;

FIG. 14C is a cross-sectional view depicting at least a portion of analternative embodiment of an exemplary Schottky diode;

FIG. 15 is a cross-sectional view depicting at least a portion of anexemplary Schottky diode, according to a third embodiment of theinvention;

FIG. 15A is a cross-sectional view depicting the gate trench structureof FIG. 15;

FIG. 15B is a cross-sectional view depicting at least a portion ofanother embodiment of an exemplary Schottky diode;

FIG. 15C is a graph illustrating a change of the conduction current foran embodiment of an exemplary Schottky diode;

FIGS. 16 and 17 are top plan and cross-sectional views, respectively,depicting at least a portion of an exemplary resistor structure in aserpentine layout, according to an embodiment of the invention;

FIG. 18 is a cross-sectional view depicting at least a portion of anexemplary capacitor structure, according to an embodiment of theinvention;

FIG. 19 is a cross-sectional view depicting at least a portion of anexemplary P-channel MOSFET, according to an embodiment of the invention;

FIGS. 20A through 20F are cross-sectional views depicting an exemplaryBiCMOS process flow, according to an embodiment of the invention;

FIGS. 21A through 21E are cross-sectional views depicting at least aportion of an exemplary BiCMOS process flow for integrating two powerdevices on the same SOI substrate, according to an embodiment of theinvention;

FIGS. 22A-22C illustrate the electric field distribution between gateand drain regions for various shielding structures;

FIG. 23 is a cross-sectional view illustrating a chip-scale assembly;

FIGS. 24A and 24B are cross-sectional views depicting at least a portionof an example N-channel LDMOS transistor formed on a bulk semiconductorsubstrate, according to an embodiment of the invention;

FIGS. 25A and 25B are cross-sectional views depicting at least a portionof an example N-channel LDMOS transistor formed in and/or on anepitaxial layer which is formed on a bulk semiconductor substrate,according to an embodiment of the invention;

FIG. 26 is a cross-sectional view depicting at least a portion of anexample N-channel LDMOS transistor formed on a bulk semiconductorsubstrate, according to another embodiment of the invention; and

FIG. 27 is a cross-sectional view depicting at least a portion of anexample N-channel LDMOS transistor formed in and/or on an epitaxiallayer which is formed on a bulk semiconductor substrate, according toanother embodiment of the invention.

It is to be appreciated that elements in the figures are illustrated forsimplicity and clarity. Common but well-understood elements that may beuseful or necessary in a commercially feasible embodiment may not beshown in order to facilitate a less hindered view of the illustratedembodiments.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the invention will be described herein in the context ofillustrative power management circuits and semiconductor fabricationmethods for forming one or more components suitable for use in theillustrative power management circuits. It should be understood,however, that embodiments of the invention are not limited to theparticular circuits and/or methods shown and described herein. Rather,embodiments of the invention are more broadly related to techniques forfabricating an integrated circuit in a manner which achieveshigh-frequency performance for a variety of power managementapplications, such as, for example, a DC/DC power converter, andadvantageously reduces the physical size and cost of external componentswhich may be used in conjunction with embodiments of the invention, suchas, for example, an output filter, among other benefits. Moreover, itwill become apparent to those skilled in the art given the teachingsherein that numerous modifications can be made to the embodiments shownthat are within the scope of the claimed invention. That is, nolimitations with respect to the embodiments shown and described hereinare intended or should be inferred.

For the purpose of describing and claiming aspects of the invention, theterm MOSFET as used herein is intended to be construed broadly so as toencompass any type of metal-insulator-semiconductor field-effecttransistor (MISFET). The term MOSFET is, for example, intended toencompass semiconductor field-effect transistors that utilize an oxidematerial as their gate dielectric, as well as those that do not. Inaddition, despite a reference to the term “metal” in the acronyms MOSFETand MISFET, a MOSFET and/or MISFET according to embodiments of theinvention are also intended to encompass semiconductor field-effecttransistors having a gate formed from a non-metal, such as, forinstance, polysilicon.

Although implementations of the present invention described herein maybe implemented using p-channel MISFETs (hereinafter called “PMOS” or“PFET” devices) and re-channel MISFETs (hereinafter called “NMOS” or“NFET” devices), as may be formed using a BiCMOS (bipolar complementarymetal-oxide-semiconductor) fabrication process, it is to be appreciatedthat the invention is not limited to such transistor devices and/or sucha fabrication process, and that other suitable devices, such as, forexample, laterally diffused metal-oxide-semiconductor (LDMOS) devices,bipolar junction transistors (BJTs), etc., and/or fabrication processes(e.g., bipolar, complementary metal-oxide-semiconductor (CMOS), etc.),may be similarly employed, as will be understood by those skilled in theart given the teachings herein. Moreover, although embodiments of theinvention are fabricated in a silicon wafer, embodiments of theinvention can alternatively be fabricated on wafers comprising othermaterials, including but not limited to gallium arsenide (GaAs), galliumnitride (GaN), indium phosphide (InP), cadmium selenide (CdSe), cadmiumtelluride (CdTe), zinc sulfide (ZnS), etc.

As previously stated, when device current is limited to a few hundredsof milliamperes (i.e., device power consumption less than about twowatts), the illustrative power stage can be monolithically integrated ina power management circuit architecture as shown in FIG. 1, wherein thecontrol circuitry 102, driver stage 104 and power switches 106, 108 areall fabricated on the same IC chip 100. However, when device powerconsumption increases beyond about five watts or so (e.g., greater thanabout two amperes (A)), an alternative partitioning of the powermanagement circuit is advantageous and/or required.

For example, FIG. 2 is a block diagram illustrating an exemplary powerstage comprising power management control circuitry 102 and a driverstage 104 implemented in a first IC 200, and power switches implementedin individually packaged discrete IC devices, 202 and 204, coupled withand external to the first IC. Unfortunately, while this solution enablesthe control circuitry to be fabricated separately from the powerswitches, and thus benefit from an ability to individually optimize thefabrication process for each IC, parasitic impedances (primarilyparasitic inductance) associated with interconnections 206 (e.g.,printed circuit traces, bond wires, ball grid array (BGA), etc.) betweenthe first (control) IC 200 and the power switch ICs 202 and 204,essentially prevent this approach from being used in high-frequencyapplications (e.g., above about one megahertz). However, this approachis generally used for power conversion in the range of about 5-30 watts.

FIG. 3 is a block diagram illustrating at least a portion of anexemplary power stage 300 comprising power management control circuitry302 implemented in a first IC 304, and a driver stage 306 and powerswitches 308 and 310 implemented in a second IC 312 coupled with thefirst IC 304, according to an embodiment of the invention. Thepartitioning of the power stage 300 as shown in FIG. 3 is applied, forexample, to DC/DC converters and other circuits and subsystems with apower conversion larger than about 30 watts. More particularly, thepower stage 300 is partitioned into a control IC 304, fabricated in amore complex digital VLSI (very large scale integration) technologyprocess, and a power block 312 implemented as a multi-chip module (MCM)including the driver stage 306, fabricated in an analog technology, anddiscrete power switches 308 and 310 integrated as bare dies in the MCM.

The MCM approach of FIG. 3 is used for power management systems forlarge power applications, e.g., for desktop computers. In this case themodule contains three separate dies: driver chip and two MOSFET powerswitches. Portable electronics puts a strong demand on miniaturizationof the implemented subsystems (i.e., small volume), and on reducingpower losses generated in power conversion stages. Thus, aspects of theinvention provide a cost-effective technology solution allowing amonolithic integration of driver stages with power switches which enablea two-die solution according to the partition scheme shown in FIG. 3.That is, the driver and FET power switches can be manufactured on thesame die for medium power applications needed for battery operatedportable electronics. There is presently no technology enabling suchsystem partitioning for a power range higher than about five watts.

Typically, a digital/analog process, such as, for example, a BiCMOStechnology, is developed with an aim to maximize integration density andspeed of signal processing. Optional power switches which can bedesigned using existing doping profiles and process steps generallycannot achieve sufficient performance in a power management application.The reduction of transistor on-resistance and the reduction of switchingpower loss require a dedicated optimization of the doping structure anduse of a tailored sequence of process steps. This is usually done in thedesign of discrete power switches only. On the other hand, theprocessing of discrete power switches does not allow a monolithicintegration of different electronic components, including NFETs, PFETs,bipolar junction transistors, P-N junction and Schottky diodes, etc.

Power management systems (e.g., DC/DC converters) typically use powerswitches to perform a high-frequency chopping of the input power and usean output filter comprising inductors and capacitors to stabilize theoutput voltage under variable load conditions. The higher the switchingfrequency, the better the power conversion performance, and smallervolume and cost of the required output filter. An increase in theswitching frequency from about 1 megahertz (MHz) available today toabout 5 MHz is desired but has not been achievable due to associatedswitching power losses in the power transistors used to implement thepower switches which are attributable, at least in part, to deviceparasitic impedances (e.g., internal capacitance, inductance, andresistance).

It is known that the switching performance of power MOSFETs can bedrastically improved by reducing internal capacitances and the chargestored in an internal body diode (see, e.g., U.S. Pat. Nos. 7,420,247and 7,842,568). FIGS. 4 and 5 are cross-sectional views depictingdiscrete laterally diffused metal-oxide-semiconductor (LDMOS)transistors 400 and 500, respectively, known in the art. The design of apower MOSFET on a silicon-on-insulator (SOI) substrate often provides asignificant technical advantage in the performance of the MOSFET. FIGS.6 and 7 are cross-sectional views depicting LDMOS transistors 600 and700, respectively, formed on SOI substrates that are known in the art.Buried oxide beneath an active SOI layer (e.g., 602 in FIGS. 6 and 702in FIG. 7) lowers output capacitance (C_(OSS)) and strongly reduces abody diode volume, thereby reducing a diode stored charge (Q_(IT)) andrelated power loss during commutation (i.e., reversing bias across thetransistor), compared to standard device structures. Both featuresreduce associated switching losses and enable an increase in theoperating frequency of the device. Despite these technical advantages ofthe use of SOI substrates, the proposed transistors have not beenbroadly adapted for manufacturing of discrete and/or integrated powerMOSFETs due to the increased cost of the product. Also, acceptance ofthis approach is impeded by problems with the long term reliability ofthe gate oxide at the gate side corner of the gate due to hot carrierinjection (HCI) under avalanche condition.

Thus, there is a need to develop an analog integration process focusedon optimal switching performance of lateral power devices, which allowsa monolithic integration of different types of power switches along withthe associated driving stages and, optionally, some monitoring andprotection functions. Power stages manufactured in accordance withaspects of the invention provide an enhanced power management solutionfor an input voltage range between about one volt and about ten volts(V), and an output current between about one ampere and about fiveamperes. Accordingly, the delivered power will cover a range roughlybetween three watts and 30 watts, although embodiments of the inventionare not limited to this or any specific power range.

As will be explained in further detail below, embodiments of theinvention described herein are based on a 20-volt BiCMOS technologyimplemented on SOI substrates with dielectric lateral isolation. Thesystem partitioning presented in FIG. 3 is achieved as a two-diesolution, according to embodiments of the invention. A chip-scaleassembly (i.e., chip-scale package (CSP) or wafer-level packaging (WLP))is preferred to avoid volume and cost associated with packaging ofindividual components. In embodiments of the invention, the higher costof the power switches is leveraged by a lower cost of integrateddrivers, and a strong reduction in volume and cost of filter componentsachieved by an increase in operating frequency.

FIG. 8 is a cross-sectional view depicting at least a portion of anexemplary structure 800 which incorporates aspects according to anembodiment of the invention. The structure 800 may be fabricated using aBiCMOS process technology on an N-type or P-type substrate 801. Withreference to FIG. 8, the structure 800 includes a combination of aburied well 802 which is locally implanted at the bottom of an activelayer 804 and a plurality of trenches (i.e., trench stripes) 806 havingsidewalls and bottom walls lined with gate oxide 808, or an alternativedielectric, and filled with polysilicon material 810, or an alternativeconductive material. Trenches 806 are preferably formed as a group ofparallel stripes which, when properly biased, affect a current flowtherebetween (e.g., in the case of a FET or Schottky diode embodiment),or which function to increase a capacitance per area of the structure(e.g., in the case of a capacitor embodiment). In this example, theactive layer 804 is formed as an N⁻ region and the buried well 802 isformed as a P⁺ well, although other embodiments may utilize analternative doping scheme (e.g., N⁻ region and N⁺ buried well, or P⁻region and P⁺ or N⁺ buried well), as will become apparent to thoseskilled in the art given the teachings herein.

The configuration of structure 800 beneficially allows integration of avariety of components, such as, for example, field-effect transistors(FETs), BJTs, PN diodes, Schottky diodes, resistors and capacitors. Eachof the trenches 806 extends substantially vertically from a top surface812 of the structure 800, through the active layer 804, and at leastpartially into the buried well 802. In alternative embodiments, thetrenches 806 may extend through the buried well 802, into the buriedoxide layer 818. The oxide lining 808 covering the sidewalls and bottomwalls of the trenches 806 prevents direct electrical connection betweenthe polysilicon material 810 filling the trenches and the buried well802. Polysilicon fill 810 is preferably used as a gate terminal whichcan be biased as in, for example, FET and Schottky diode embodiments.

The buried well 802 has an important function in devices operative tosustain an applied blocking voltage, such as transistors or diodes. Moreparticularly, a doping level, doping type and/or a location of theburied well 802 are configured in a manner which substantially pins(i.e., clamps) a breakdown voltage at the PN junction created between anupper right side (i.e., tip) of the buried well and an N⁻ backgrounddoping of the active layer 804. By selectively controlling one or morecharacteristics of the buried well 802, an electric field distributionin the device is controlled.

The trench stripes 806 having walls (i.e., sidewalls and bottom walls)lined with gate oxide 808 are placed between main terminals of the powerdevices formed therein. The term “main terminals” as used herein isintended to broadly refer to external connections to the device, suchas, for example, source and drain terminals, in the case of an MOSdevice, or anode and cathode terminals, in the case of a diode. Thetrench gates stripes 806 are formed (e.g., etched) substantially inparallel to a current path in the illustrative embodiment shown in FIG.8. As a result, a conduction current flows in the N⁻ active layer 804between the gate trenches 806 and can be controlled (e.g., modulated) byan applied gate potential, in the case of, for example, a lateralSchottky diode. In the case of a FET structure formed in accordance withone or more embodiments of the invention, the trench gates 806 areoperative to deplete or enhance a gate/body interface, controlling thecurrent flow through an inversion channel formed in the device.

Doped polysilicon material 810 filling the trenches is used to create agate bus connecting the gate regions to a gate terminal in a thirddimension (not explicitly shown). For an NFET device formed according toan embodiment of the invention, the polysilicon material 810 ispreferably doped with phosphorous, with a doping concentration ofgreater than about 10¹⁹/cm³, while for a PFET device, the polysiliconmaterial is preferably doped with boron having a doping concentration ofabout 10¹⁹/cm³. The top surface of polysilicon gate layer 810 is shownoptionally covered by a layer of silicide material 814 (e.g., titaniumsilicide (TiSi) or tungsten silicide (WSi)) with low resistivity, whichcan be deposited thereon using a known silicide deposition process(e.g., chemical vapor deposition (CVD), sputter deposition, etc.). Thesilicide layer 814, which forms a polycide electrode in the structure800, reduces a gate resistance of the structure.

In a preferred embodiment, narrow gate trenches 806 are formedunderneath the polycide electrode 814 along a path of current flow inthe active layer 804. In this manner, the trenches 806 increase aneffective gate width in the MOSFET structure 800, among otheradvantages.

Another trench structure 816, formed deeper than trenches 806, ispreferably used to create a lateral isolation region between integratedcomponents. The deep trench structure 816, also referred to herein as alateral isolation trench, can be formed, for example, by etching fromthe top surface 812 of the structure, through the active layer 804, to aburied oxide layer 818 formed on the substrate 801. The lateralisolation trench 816 can be filled with oxide, or a combination of oxideand polysilicon. An optional deep trench cut (i.e., etch), notexplicitly shown, through the buried oxide layer 818 to the substrate801 can be used as a substrate contact. This optional trench ispreferably filled with doped polysilicon, or an alternative conductivematerial, to ensure good ohmic (i.e., low resistance) contact to thesubstrate 801.

A variety of electronic components can be created using an illustrativeBiCMOS process flow, according to embodiments of the invention. Examplesof some components which can be formed which incorporate aspects of theinvention are described herein below with reference to FIGS. 9A through19.

FIG. 9A is a cross-sectional view depicting at least a portion of anexemplary N-channel LDMOS transistor 900, according to an embodiment ofthe invention. The LDMOS transistor 900 has reduced gate-to-draincapacitance (C_(gd)) in comparison to standard LDMOS devices, due atleast in part to the effect of the gate shield layer. Moreover, LDMOStransistor 900 shows a small impact of the reverse recovery of the bodydiode (Q_(rr)) due at least in part to reduced diode stored charge.Transistor 900 includes an integrated PN clamping diode (i.e., a diodeformed by the end of the deep well 902 and the drain region) as anintegral part of each active cell that pins the avalanche breakdown awayfrom the gate oxide and close to the upper right corner of a buried P⁺well 902. That is, under blocking condition, the avalanche impactionization is localized between the tip of the buried well 902 and thecorner of the drain contact region, within the volume of the activelayer and away from the top and bottom oxide interfaces. This increasesthe avalanche ruggedness of the power transistor without causing anyreliability issues. This transistor design minimizes the injection ofhot carriers into oxides, improving the long term reliability of thepower switch. A conduction current flows from a source region 905between walls of a trench gate 906 into a lightly doped drain (LDD)extension region 908 into a drain contact 910. An alternative view of asimilarly-formed trench gate (with the cross-section taken through thetrench) is shown as structure 906 depicted in FIG. 9B. FIG. 9Billustrates the trench gate 906 formed substantially vertically througha P-type body region 920 and into the buried P⁺ well 902 formed at thebottom of the body region. The trench gate 906 has walls (i.e.,sidewalls and bottom walls) lined with gate oxide 922. Also shown inFIG. 9B is a lateral isolation structure 924, which may be formed inmanner consistent with the lateral isolation structure 816 shown in FIG.8, that provides isolation between integrated components. Duringprocessing, a P⁻ handle wafer in SOI substrate gets depleted along a P⁻substrate/buried oxide interface, which reduces an output capacitance,C_(oss), of the MOSFET.

It is to be appreciated that, in the case of a simple MOS device,because the MOS device is symmetrical in nature, and thus bidirectional,the assignment of source and drain designations in the MOS device isessentially arbitrary. Therefore, the source and drain regions may bereferred to generally as first and second source/drain regions,respectively, where “source/drain” in this context denotes a sourceregion or a drain region. In an LDMOS device, which is generally notbidirectional, such source and drain designations may not be arbitrarilyassigned.

The buried well 902, like the buried well 802 shown in FIG. 8, has animportant function, especially in devices operative to sustain anapplied blocking voltage (e.g., transistors and diodes). Moreparticularly, a doping level, doping type and/or a location of theburied well 902 are configured in a manner which substantially clampsthe breakdown voltage at the PN junction formed between an upper rightside of the buried well and an N⁻ background doping of an active layer904 in the device. By selectively controlling one or morecharacteristics of the buried well 902, an electric field distributionin the device is controlled. For instance, the device can beadvantageously arranged such that a maximum electric field isdistributed between the upper right corner of the buried well 902 and aright bottom corner of a drain contact region 910. When configured inthis manner, a clamping PN diode is integrated within the device whichkeeps hot carriers, generated by avalanche impact ionization, far awayfrom a top silicon/oxide interface. This feature increases an ability ofthe device to absorb avalanche energy without creating reliabilityissues in the device.

When the illustrative SOI LDMOS transistors 600 and 700 shown in FIGS. 6and 7, respectively, are pushed into avalanche, impact ionization willtake place at a bottom corner of the gate covering a lightly doped drain(LDD) region in the device, and the injection of hot carriers into agate oxide in the device will often result in reliability issues, as areknown to those skilled in the art. For at least this reason,conventional LDMOS structures on SOI are not suitable for use as powerswitches. By providing an ability to clamp the avalanche at a desiredlocation in an LDMOS transistor device, the LDMOS structure formed inaccordance with one or more aspects of the invention is well-suited foruse in a power switching application.

With continued reference to FIGS. 9A and 9B, LDMOS transistor 900includes a shield field plate 912, or alternative shielding structure,which, in this embodiment, is formed as a lateral extension of aconductive layer lining the source trench contact walls, overlaps a gate(e.g., polysilicon structure) 914 and comes into close proximity with anoxide interface along the N drain extension region (i.e., LDD region)908. The conductive layer is preferably deposited as a titanium(Ti)/titanium nitride (TiN) stack, but may be also formed of othermaterials, such as, for example, a titanium (Ti)/tungsten silicide (WSi)film. In this illustrative embodiment, the source trench is formed onthe left-hand side of the LDMOS transistor 900, having side walls and abottom wall lined with gate shield plate 912 and filled with top metal.

The shield 912 functions primarily as a field plate, distributing (e.g.,stretching) an electric field distribution along a top oxide interfaceaway from an edge (e.g., bottom right corner) of the gate 914 nearestthe drain, and also as a shield that helps to reduce gate-to-draincapacitance, C_(gd) (so called Miller capacitance, which determines theswitching speed of the transistor), at a positive bias of the drain andfurther improves gate oxide reliability. The electric field peakappearing at the drain side corner of the gate 914 is now split betweenthe gate corner and the end of the field plate, reducing the electricfield peak value and inhibiting early injection of hot carriers into theoxide. Drain and source contacts 910 and 916, respectively, are formedas metal-filled vias reaching a patterned top metal layer (notexplicitly shown, but implied) and form drain (D) and source (S)terminals, respectively, of the LDMOS transistor 900. Depleting thelightly doped drain extension region (908) also at a positive biasapplied to the drain contact 910 also helps reduce C_(gd). A silicidelayer 918 formed on the polysilicon gate structure 914, thereby forminga polycide layer (also referred to as silicided polysilicon), is used tocreate a gate bus leading to a gate terminal (G) located in a thirddimension (not explicitly shown, but implied). The silicide layer 918 ispreferably formed using a known deposition process (e.g., CVD,sputtering, etc.).

FIG. 10 is a cross-sectional view depicting at least a portion of anexemplary N-channel LDMOS transistor 1000, according to anotherembodiment of the invention. This LDMOS transistor 1000 is designed as asimplification of the LDMOS transistor 900 shown in FIGS. 9A and 9B. Asapparent from FIG. 10, one simplification in the fabrication of LDMOStransistor 1000, compared to LDMOS transistor 900 shown in FIGS. 9A and9B, comprises removal of the gate trenches (906 in FIGS. 9A and 9B). Aprimary impact on the performance of the MOSFET 1000 is a smaller gatewidth per unit area, which increases on-resistance, R_(ON), of theresulting device. This can be leveraged by making the channel lengthshorter, as the alignment restriction related to an overlap of the gatepolysilicon over gate trench endings is removed. Other features andcharacteristics of the MOSFET transistor 1000 remain essentially thesame as for LDMOS transistor 900.

FIG. 10A illustrates an alternative embodiment of the LDMOS transistor1000 of FIG. 10. Specifically, this embodiment includes a modificationto extend the breakdown voltage above 20V. In the LDMOS transistor 1000Aof FIG. 10A, a narrow stripe of thicker oxide 1016 is introduced betweenthe gate and the drain terminals. This allows the gate shield 1012 toform a two-step field plate (i.e., raised step portion 1014 is added),which further improves the electric field distribution.

FIGS. 22A-22C show the electric field distribution between gate anddrain regions in various configurations. FIG. 22A shows the electricfield distribution between gate and drain regions without the effect ofthe field plate. The high electric field peak at the corner of the gateallows the injection of hot carriers into the gate oxide, which reducesthe reliability of the transistor. FIG. 22B shows that the field plateformed by the gate shield layer, in a transistor designed for 20Vbreakdown, eliminates this critical electric field. Further improvementof the field plate in the shielding structure is shown in FIG. 22C wherethe introduction of a thicker oxide stripe enables a two-step fieldplate contour that pushes the peak electric field away from the gatecorner. As noted above, this structure is preferably used in transistorsdesigned for a breakdown voltage higher than 20V.

FIG. 11 is a cross-sectional view depicting at least a portion of anexemplary low voltage signal MOSFET 1100, according to an embodiment ofthe invention. The MOSFET 1100 includes a P body region 1102 and an Ndrain region 1104, which can be used to form other circuit components,as will be described in further detail below. In this embodiment, the P⁺buried well is not directly connected with the source terminal, as it isin the exemplary MOSFET 1000 shown in FIG. 10, but rather is connectedwith a separate bulk (B) terminal. This configuration allows a voltagepotential to be applied to the buried well that is different from thevoltage potential applied to the source terminal. The MOSFET 1100 isformed by a further simplification of the illustrative transistor 1000shown in FIG. 10. Specifically, a pitch of the basic cell has beenreduced, thereby allowing a higher density of such devices to be placedin the circuit. As a trade-off for higher density, MOSFET 1100 hasreduced high-voltage capability and reduced avalanche ruggedness, butthese features are generally more important for power switchingapplications.

In certain embodiments of the MOSFET described above in connection withFIGS. 9A to 11, the transistor is designed to a target breakdown voltagespecification between 12V and 60V. These MOSFETs are optimized forapplication as power switches with minimized conduction and switchingpower losses in low voltage power management systems. The embodimentsdescribed herein enable an increase of the switching frequency of DC/DCconverters from 1.5 MHz to 5 MHz while implementing the integratedscheme of FIG. 3.

With reference now to FIG. 12A, a cross-sectional view depicts at leasta portion of an exemplary SOI bipolar junction transistor (BJT) 1200,according to an embodiment of the invention. BJT 1200 is formed as amodification of the MOSFET transistor 1000 shown in FIG. 10. Here, aformer body region 1102 of the MOSFET 1100 shown in FIG. 11 is used as abase region 1201 of the BJT 1200. The source trench contact has beenremoved. Instead, an emitter contact 1202 is cut across the base region1201 to make a connection between a deep P⁺ layer 1204 and the polycidestructure (i.e., silicided polysilicon). FIG. 12B depicts an exemplaryBJT 1250 illustrating one way to form the connection between the deep P⁺layer 1204 and the polycide structure, according to an embodiment of theinvention. Specifically, a connection 1252 between the deep P⁺ layer1204 and the polycide structure 1254 is formed as small spots (i.e.,contacts) by interrupting the emitter contact 1202 (e.g., betweenfingers) along the finger layout. The connection 1252, in thisembodiment, is formed as a lateral extension of the titanium(Ti)/titanium nitride (TiN) layer (in a manner similar to the fieldplate 912 shown in FIG. 9) and overlaps the polycide structure 1254.These contacts are preferably placed at prescribed intervals along thepolycide stripe (e.g., the polycide region which, in the case of aMOSFET, would be the gate patterned as a stripe, and is used to build abase bus contact), and the polycide layer is used to create a base buswith low resistivity. Using the polycide material as the current buscontacting the base region to the base terminal assures low baseresistance, which improves the switching performance of the transistor.

FIG. 12C illustrates a BJT embodiment 1250A where a base contact 1256 isformed partially in a trench region formed in the active region and inelectrical contact with the connection 1252.

In an embodiment illustrated in the partial cross-sectional view of FIG.12D, the connection 1252A does not directly contact the polycide basebut rather is spaced therefrom. A button-like contact 1258 is providedand contacts both the upper surface of the polycide base and the suppersurface (and optionally side surface) of connection 1252A. One or moreof these button contacts 1258 can be spaced (in a third dimension alongthe base polycide stripe and interleaved with the emitter contact) tomake contact between the polycide base structure with the deep P+ well1204 and base contact 1256 via the connection layer 1252A.

The initial high-voltage capability and the avalanche ruggedness of theMOSFET transistor are preserved using this BJT configuration. The PNjunction created between the tip of the deep P+ well and the collectorregion acts as the clamping diode. As in the case of the MOSFET, the PNclamping diode of the BJT pins the area of the avalanche breakdownwithin the volume of the silicon layer, confining hot carriers generatedby avalanche impact ionization to this location.

With reference to FIG. 12A, one can observe that the BJT includes a MOSchannel underneath the polycide stack, which creates a MOSFET structurein parallel with the bipolar transistor. If the positive bias applied tothe BJT-base terminal is larger than the threshold voltage of theMOSFET, then the collector current is increased by the electron currentflowing through the MOS channel, which improves the gain ((3) of theBJT.

Though not shown, a shield structure as discussed in connection withother embodiments described herein may also be incorporated into thisdesign to improve the breakdown/reliability performance of the BJTdevice.

As shown in FIG. 12E, the NPN BJT transistor structure shown in FIG. 12Acan be converted into a PNP BJT by reversing the polarity of theimplanted dopant regions.

As noted above, the basic MOSFET structure can be adapted to providepower diodes. Unlike conventional power PN diodes designed in a VLSItechnology, the power PN diodes described herein exhibit avalancheruggedness. Moreover, the basic MOSFET structure can be adapted toprovide Schottky diodes, which are typically not in the designer's VLSIcomponent toolbox. The structure disclosed herein increases theflexibility of the power management IC design by implementation of PNand Schottky diodes able to sustain the full supplied voltage. Thesediodes exhibit unique avalanche ruggedness when voltage spikes in thesupply voltage rail drive the circuit beyond the allowed maximumblocking voltage value. These proposed diodes are compatible with theprocess flow disclosed for the SOI-MOSFETs and are straightforwardmodifications of that structure.

FIG. 13 is a cross-sectional view depicting at least a portion of anexemplary PN diode 1300, according to an embodiment of the invention.The PN diode 1300 is obtained as a modification of the exemplary MOSFETtransistor 1000 shown in FIG. 10. Here, the N+ source region has beenomitted, and the PN junction used to form the diode 1300 is created by ajunction of the former P body 1102 and N drain 1104. An anode (A)terminal is formed having a trench contact 1302 adapted for electricalconnection with the P body 1102. A cathode (C) terminal is adapted toprovide electrical connection with N drain 1104 of the diode. Theinitial high-voltage capability and the avalanche ruggedness of theMOSFET structure are preserved.

FIG. 13A is a cross-sectional view depicting at least a portion of anexemplary embodiment of a PN diode 1300A, according to an alternativeembodiment. The PN diode 1300A is identical to the PN diode 1300 exceptfor the modified trench contact 1302A, which extends to at leastpartially overlap the gate to provide a shielding structure including afield plate, as described above in connection with FIGS. 9A and 10. ThePN diode of FIG. 13A preserves the full voltage blocking capability ofthe LDMOS structure by the presence of the deep P⁺ well placed along theactive layer interface to the buried oxide. Under blocking condition,the avalanche impact ionization is localized between the tip of theburied well and the corner of the cathode contact region, within thevolume of the active layer and away from top and bottom oxideinterfaces. This design of the diode minimizes the injection of hotcarriers into oxides improving the long term reliability of the powerdevice. The PN junction, which is created by the end of the deep P⁺ welland the cathode region, acts as a clamping diode as an integral part ofeach active cell of the diode. The avalanche breakdown defined by thisclamping diode is pinned within the volume of the Silicon materialincreasing the avalanche ruggedness of the power device. The integratedshielding structure reduces the capacitive coupling between the gate andcathode terminals and serves as a field plate stretching out theelectric field distribution along the top oxide interface with the Lddregion. The electric field peak appearing at the cathode side corner ofthe gate stack is now split between the gate corner and the end of thefield plate, reducing the electric field peak value and inhibiting earlyinjection of hot carriers into the oxide.

In the embodiments of FIGS. 13 and 13A, the polycide gate and the deepP+ well can be connected to the anode terminal in a third dimension. Asshown in the partial cross-sectional view of FIG. 13B, the polycide gatecan be connected to the anode and deep P+ well via a button-like contact1304 extending through the conductive layer/gate shield 1302A that makeselectrical contact with both the polycide of the gate structure and withthe conductive layer/gate shield 1302A (e.g., by being formed partly onthe top surface of the conductive layer 1302A). One or more of thesebutton contacts 1304 can be spaced (in a third dimension) to makecontact to the gate polycide in prescribed intervals (e.g., in a fingerlayout) along the polycide stripe rather than interrupting the shieldingstructure 1302A with a continuous contact. FIG. 13C illustrates analternative approach to coupling the gate polycide to the anode and deepP+ well. Specifically, the anode trench contact 1302B has a lateralextension that (like the extension shown in FIG. 12B) that extends toconnect the gate polycide to the anode and deep P+ well. This extension1302B also extends to form the shielding structure with field plate asdescribed above in connection with the embodiment FIG. 13B. In anotherembodiment, the gate bus can be configured as a terminal separate fromthe anode, and the two can be shorted together externally. This approachmay provide for simpler processing in some embodiments.

Monolithic integration of Schottky diodes has been observed for powerMOSFETs where the Schottky diode clamps the integral body diode. Thisapproach is aimed at avoiding power losses related to the stored chargeQrr during commutation of the drain-to-source bias of the power MOSFET.In the approach of U.S. Pat. Nos. 6,049,108 and 6,078,090, a Schottkydiode is integrated within a trench-MOSFET structure where the proximityof two trench walls below the Schottky contact is used to shield theSchottky contact interface from a high electric field induced by thedrain voltage under blocking condition. The advantage of this TMBSstructure (Trench-MOS-Barrier-Schottky) is that the electrical shield ofthe Schottky contact enables the use of the higher doping of thesemiconductor (lower Vf) without any deterioration of the blockingcapability. Also, the leakage current of a TMBS diode has flat voltagecharacteristics up to the breakdown voltage defined by the PN junctionpresent in a neighbor cell.

U.S. Pat. No. 7,745,846 discloses a Schottky diode integrated as adedicated cell in an LDMOS transistor structure. The structure has avertical current flow towards the drain contact at the back side of thewafer. The electrical shielding of the Schottky contact formed betweenthe top metal and the LDD-1 region is achieved by the blocking impact ofthe gate and P-buffer regions. The forward I-V characteristics of theSchottky diode can be influenced by the gate potential. The integratedSchottky diode has the same blocking voltage capability as the parentLDMOS transistor. No comparable Schottky diodes are proposed in the artfor power management ICs.

FIG. 14A is a cross-sectional view depicting at least a portion of anexemplary Schottky diode 1400, according to an embodiment of theinvention. The Schottky diode 1400 is formed as a modification of the PNdiode 1300 shown in FIG. 13. Specifically, the anode trench contact 1302is omitted, along with the P body region 1102 (see FIG. 13), allowing aSchottky barrier to be created at an interface between the anode contact(metal) 1402 and an N⁻ active layer 1404. In embodiments, the Schottkybarrier is stabilized by a two-step rapid thermal annealing (RTP at 650°C. and 820° C.), which results in a formation of a silicide phase (e.g.,TiSi₂) at the contact interface. The top polycide layer 1406 and deep P⁺well 1408 are electrically connected to the anode (A) terminal andinduces a pinching of the electric field distribution under an appliedblocking bias of the cathode (C) terminal. This pinching effect createdby the placement of the gate stack at the top of the N− region and thedeep P+ well at the bottom of the N− region, which is similar to anaction of a JFET channel, shields the Schottky contact interface againstany high electric field under blocking conditions. The shielding effectkeeps leakage current in the diode 1400 low in the full range of theblocking voltage (e.g., about 12 volts to about 20 volts, although theinvention is not limited to any specific voltage or range of voltages).The value of the leakage current in the diode will be a function ofdoping characteristics of the N⁻ active region 1404 at the Schottkycontact, as will be known by those skilled in the art. (See, e.g., U.S.Pat. No. 5,365,102, the disclosure of which is incorporated by referenceherein in its entirety.). Shielding of the Schottky contact enables ahigher doping of the semiconductor at the Schottky interface, asdiscussed below in connection with FIG. 14B, and improves the electricalperformance of the diode.

The connection of the deep P⁺ well 1408 and the polycide structure 1406with the anode contact 1402 can be formed in a manner consistent withthe connection 1252 shown in FIG. 12B for the illustrative BJT device1250. Specifically, the connection between the deep P⁺ layer 1408 andthe polycide structure 1406 is preferably formed as small spots byinterrupting the anode contact 1402 along a finger layout. Theconnection, in this embodiment, is formed as a lateral extension of thetitanium (Ti)/titanium nitride (TiN) layer (not explicitly shown in FIG.14A, but implied in a manner similar to the connection 1252 shown inFIG. 12B) and overlaps the polycide structure 1406. These contacts arepreferably placed at prescribed intervals along the polycide stripe(e.g., the polycide region which, in the case of a MOSFET, would be thegate patterned as a stripe), and the polycide layer is used to create alow resistivity shielding structure which is operative to shield theSchottky contact against any high electric field under blockingconditions, as previously stated.

FIG. 14C illustrates a cross-sectional view depicting at least a portionof an exemplary Schottky diode 1400A, according to an alternativeembodiment. The Schottky diode 1400A is identical to the Schottky diode1400 except for the inclusion of the shielding structure 1414, whichserves as both a gate shield and a field plate for the Schottky diode1400A. The connection of the deep P⁺ well 1408 and the polycided gatestructure 1406 with the anode contact 1402 can be formed in a mannerconsistent with the connection shown in FIG. 13B or 13C for theillustrative PN diode device 1300A, or external connections can be madeas dictated by the process. In embodiments, the shielding structure 1414can also be incorporated into the Schottky diode structure shown in FIG.14B.

The initial high-voltage capability and the avalanche ruggedness of theMOSFET structure are preserved, as the blocking voltage is sustained bythe device structure on the cathode (former drain) side of the toppolycide electrode, and the avalanche breakdown is clamped by the PNjunction at the upper right corner (i.e., tip) of the deep P⁺ well 1408.The deep well 1408 is preferably an implanted well with a maximum dopingconcentration close to the Si/buried oxide interface. In a preferredembodiment, the maximum doping concentration is in the range of about5e16 cm⁻³ and 5e17 cm⁻³, and the doping profile is configured to slopedown towards the surface. It is to be appreciated, however, that theinvention is not limited to a specific doping concentration or profileof the deep well 1408. The PN junction, in this embodiment, is formed bythe deep P⁺ well 1408, N⁻ active layer 1404, N region 1410 and N⁺ region1412 toward the cathode terminal.

FIG. 14B is a cross-sectional view depicting at least a portion of anexemplary Schottky diode 1450, according to another embodiment of theinvention. The Schottky diode 1450 is essentially the same as theSchottky diode 1400 depicted in FIG. 14A, except that an additional Nregion 1452 is formed in the N⁻ active layer 1404, proximate an uppersurface of the N⁻ active layer, in a manner similar to N region 1410. Anadvantage of the Schottky diode 1450, compared to Schottky diode 1400shown in FIG. 14A, is that the forward voltage drop of the Schottkydiode 1400 depicted in FIG. 14A can be reduced by increasing the dopingconcentration of the N⁻ active layer 1404 proximate the Schottkycontact. In a preferred embodiment, this is achieved by extending the Nimplant region 1410 at the cathode side of the polysilicide region tothe region under the anode (A) contact 1402, shown as N region 1452 inFIG. 14B.

As described herein above in conjunction with the exemplary structuresdepicted in FIGS. 10 through 14C, an important benefit according to oneor more embodiments of the invention is the inclusion of the deep wellwhich is configured to clamp the breakdown voltage away from thesilicon/oxide interface. This arrangement advantageously enables thestructure to absorb avalanche energy without experiencing reliabilityissues. Additional structures according to other embodiments of theinvention incorporate a similar configuration of the drain region, thusinheriting the avalanche ruggedness of the parent MOSFET design shown,for example, in FIGS. 9A and 9B.

FIG. 15 is a cross-sectional view depicting at least a portion of anexemplary Schottky diode 1500, according to another embodiment of theinvention. Schottky diode 1500 is formed as a modification of theillustrative Schottky diode 1400 depicted in FIG. 14A. Specifically, ina manner consistent with the modification of the MOSFET 900 shown inFIG. 9, gate trenches 1502 are formed in the device, preferably along acurrent flow path in the N⁻ mesa region (i.e., active layer) 1404underneath a polycide electrode 1504 formed on an upper surface of theactive layer of the diode 1500. The gate trench structure 1502additionally improves the shielding effect of the Schottky contactagainst the blocking voltage applied to the cathode (C) terminal. In theSchottky diode 1500, the gate electrode is decoupled from an anode (A)terminal and can be used to further modify a conduction path between thegate trenches 1502. The anode terminal is connected to the deep P⁺ well1408 in a third dimension, which is not explicitly shown but is implied.The Schottky diode 1500 may be referred to herein as a switched Schottkydiode and represents a new type of power device, according to anembodiment of the invention.

FIG. 15A is a cross-sectional view depicting the trench structure of theswitched Schottky diode device of FIG. 15 in more detail. The currentflows under conduction condition between the gate trenches 1502 from theanode (source) contact to the cathode (drain) region. If the trenchwidth is smaller than the width of the N⁻ mesa region (i.e., activeregions 1404) between the trenches 1502, then the effective gate widthper active cell is increased. The increase of the gate width correspondsto an increase of the transconductance of the MOSFET, and respectivelyto a reduction of the on-resistance of the transistor. As shown in FIGS.15 and 15A, the gate trenches are implemented in the SOI-Schottky diodestructure to further intensify the shielding effect by combination ofvertical and lateral depletion of the N− layer 1404. The gate region canbe accessed as a third terminal to turn on and off the lateral depletioneffect by applying a corresponding bias to this electrode. The anodeterminal can be connected to the deep P+ well 1408 in the thirddimension as discussed above in connection with other embodiments.Lateral isolation regions 1514, which may comprise an oxide or otherdielectric material, are formed in the diode structure 1500 toelectrically isolate the diode from other circuit components on the die.

FIG. 15B illustrates an alternative embodiment of a switched Schottkydiode 1500A. The switched Schottky diode 1500A is essentially the sameas the switched Schottky diode 1500 depicted in FIG. 1, except that anadditional N implant region 1552 is formed in the N⁻ active layer 1404,proximate an upper surface of the N⁻ active layer, in a manner similarto N region 1510. That is, the active layer is doped to a higher Ndopant concentration in the vicinity of the Schottky contact whilekeeping the doping of the active layer between the gate trenches 1502 atthe original lower N⁻ level. An advantage of the switched Schottky diode1500A, compared to switched Schottky diode 1500 shown in FIG. 15, isthat the forward voltage drop (Vf) of the Schottky diode 1500 depictedin FIG. 15 can be reduced by increasing the doping concentration of theN⁻ active layer 1404 proximate the Schottky contact, and the shieldingeffect remains unaffected. In a preferred embodiment, this is achievedby extending the N implant region 1510 at the cathode side of thepolysilicide region to the region under the anode (A) contact, shown asN region 1552 in FIG. 15B. While in known devices such as disclosed inU.S. Pat. No. 7,745,846, the forward characteristics could be modifiedby the gate-to-anode bias applied to the gate electrode, the diodestructure from FIG. 15B exhibits a change of the conduction current byfour orders of magnitude when turning on and off the gate bias, asillustrated in FIG. 15C. This switched Schottky diode may be called aπ-Switch due to the gate trench structure cross-section illustrated byFIG. 15A. Though not shown, a gate shield structure as discussed inconnection with other embodiment may also be incorporated into thisdesign to improve the breakdown/reliability performance of the device.

FIGS. 16 and 17 are top plan and cross-sectional views, respectively,depicting at least a portion of an exemplary resistor structure 1600 ina serpentine layout, according to an embodiment of the invention. Theresistor path 1602 is defined by an N⁻ region 1604 between gate trenches1606, which is connected to N⁺ contact regions 1608 and 1610 on bothends of the serpentine. One of the N⁺ contact regions, e.g. 1608,includes a trench contact 1612 to a deep P⁺ well 1603 (not explicitlyshown in FIG. 16, but shown in FIG. 17 as well 1702). The P⁺ well 1603isolates the N⁻ resistor path 1602 from a bottom, as shown in thecross-sectional view of FIG. 17. As shown in FIG. 17, buried P⁺ deepwell 1702 is operative to electrically isolate the resistor formed bythe N⁻ region between the trenches. Lateral isolation regions 1614,which may comprise an oxide or other dielectric material, are formed inthe resistor structure 1600 to electrically isolate the resistor fromother circuit components on the die.

With reference now to FIG. 18, a cross-sectional view depicts at least aportion of an exemplary capacitor structure 1800, according to anembodiment of the invention. The capacitor structure 1800 can have aserpentine layout similar to the resistor structure 1600 shown in FIG.16, or it may comprise multiple parallel stripes formed by the trenches1802. Capacitor electrodes 1802 are formed by the polysilicon fill inthe gate trenches and by the deep N⁺ well 1804 at the bottom of theactive layer 1806. Both regions are connected to terminals at ends ofthe serpentine layout, not explicitly shown but implied. Lateralisolation regions 1808, which may comprise an oxide or other dielectricmaterial, are formed in the capacitor structure 1800 to electricallyisolate the capacitor from other circuit components on the die.

FIG. 19 is a cross-sectional view depicting at least a portion of anexemplary P-channel MOSFET 1900, according to an embodiment of theinvention. The MOSFET 1900 is formed as a modification of the N-channelMOSFET 900 shown in FIG. 9, wherein a polarity type of the material usedto dope the body (P body in FIG. 9), as well as source and drainregions, have been reversed to create a P-channel LDMOS transistor. Theimplants dedicated to form a P-channel MOSFET in parallel to theN-channel transistor increases the mask count compared to the processused to make the N-channel LDMOS transistor 900 only, as will be knownby those skilled in the art. As with the N-channel LDMOS transistor1000, one simplification in the fabrication of LDMOS device 1900comprises removal of the gate trenches 1902. A primary impact on theperformance of the resultant MOSFET is a smaller gate width per unitarea, which increases on-resistance, R_(ON), of the resulting device.This can be leveraged by making the channel length shorter, as thealignment restriction related to an overlap of the gate polysilicon overgate trench endings is removed.

The exemplary electronic components depicted in FIGS. 9 through 19 canbe used to build a BiCMOS circuit including power switches, diodes, andsome associated circuitry. The BiCMOS process flow includes a basic maskset allowing manufacturing of components presented in FIGS. 9 through17, and an additional mask subset allowing the component portfolio toinclude the structures shown in FIGS. 18 and 19. As used herein, thephrase “basic mask set” is defined broadly to refer to a minimum numberof mask levels required to fabricate a set of devices based on an NFETstructure according to embodiments of the invention.

With reference now to FIGS. 20A through 20F, cross-sectional views,collectively, depict an exemplary BiCMOS process flow, according to anembodiment of the invention. The process flow uses a basic mask set formanufacturing circuit components based on modifications of the N-channelLDMOS device shown in FIG. 9, as described herein above. The process isbased on an SOI substrate with a P⁻ handle wafer, and an N⁻ activelayer. By way of example only and without limitation, an illustrativeprocess flow in accordance with an embodiment of the invention includesthe following primary steps:

Form a lateral dielectric isolation, also referred to as lateral trenchisolation (LTI), by etching a trench through an active layer 2002, andfilling the trench with oxide or a combination of oxide and polysiliconusing a first mask step (LTI mask), as shown in FIG. 20A;

Deposit a thick field oxide and pattern it with the active area mask(active mask);

Deep implantation of boron, or an alternative dopant, to form a localdeep P⁺ well 2004, or alternatively an N⁺ well as a function of thedopant employed, with a concentration peak close to an interface betweenthe P⁺ well (buried layer (BL)) 2004 and a buried oxide 2006 using asecond mask step (deep well mask), as shown in FIG. 20A;

Pattern a mask to define a position of one or more gate trenches 2008through the active layer 2002 into the buried well 2004 using a thirdmask step (trench gate mask), as shown in FIG. 20B; etch the gate trenchwith rounded bottom and top corners, grow a thermal gate oxide on thesidewalls and bottom wall of the gate trench, and fill the trench withpolysilicon 2010, not explicitly shown but implied in FIG. 20B; in analternative embodiment, the steps for forming the gate trench can beomitted, thereby simplifying the NFET structure shown in FIG. 9 to formthe structure shown in FIG. 10.

Dope the polysilicon 2010 by phosphor implantation, or an alternativedopant, and anneal, and deposit a silicide layer 2012 on the top, asshown in FIG. 20B;

Pattern the polycide layer 2012 to form a gate structure using a fourthmask level (polysilicon mask), as shown in FIG. 20B;

Implant boron to create a body region 2014 self-aligned to the edge ofthe polycide layer 2012 using a fifth mask step (body mask). Performbody diffusion, for example with a dedicated thermal anneal, as shown inFIG. 20C;

Implant phosphor or arsenic, or an alternative dopant, to create alightly doped drain (LDD) extension 2016 at the other edge of thepolycide layer 2012, opposite the edge used to form the body region2014, using a sixth mask step (LDD mask), as shown in FIG. 20C;

Create highly-doped source region 218 and drain region 220 in the bodyregion 2014 and LDD extension 2016, respectively, by shallow arsenicimplantation using a seventh mask step (source/drain mask), as shown inFIG. 20D;

Deposit field oxide 2022 over a top surface of the structure to assure apre-defined spacing of a field plate 2024 from the surface of the drainextension region 2016 as shown in FIG. 20E;

Etch a shallow source contact trench 2026 using an eighth mask step(trench contact mask), and implant BF₂ through the trench bottom (plugimplant) to assure a good ohmic contact to the body and deep P⁺ regions,as shown in FIG. 20E;

Deposit and sinter a silicide film 2028 (e.g., Ti/WSi_(x) or Ti/TiN)lining the trench contact walls to create an electric short betweensource and body regions, as shown in FIG. 20E. During the sinteringprocess, a silicide (e.g., TiSi_(x)) is created at the Si/Ti interface.Such a contact formation methodology is well known to those skilled inthe art;

Pattern the contact silicide layer allowing a lateral extension tooverlap the gate structure and create a field plate in the proximity ofthe LDD/oxide interface using a ninth mask step (field plate (FPL)mask), as shown in FIG. 20E;

Deposit an interlayer dielectric film (ILD) 2030 and apply achemical-mechanical polishing step (CMP), or an alternativeplanarization process, to achieve a substantially planar top surface, asshown in FIG. 20F;

Etch via openings to access source, drain and gate contact areas using atenth mask step (via mask). Fill vias with tungsten plugs (Ti/TiN/W), oran alternative conductive material, and apply a CMP step to planarizethe top surface again, as shown in FIG. 20F; and

Deposit and pattern a thick aluminum layer 2032 to create top electrodeswith source, drain, and gate bus structures using an eleventh mask step(metal mask), as shown in FIG. 20F.

As discussed above, the processing of an N-channel LDMOS (NFET)transistor, in this embodiment, requires eleven mask levels (i.e.,steps). The number of mask levels can be reduced to ten if the gatetrench processing is omitted, as noted above. An optional mask can beused to create an electrical contact to the substrate by etching a deeptrench through the active layer and the buried oxide, and filling itwith oxide and doped polysilicon.

In order to create a P-channel MOSFET (PFET) using the same processflow, an additional mask subset is required. According to anillustrative embodiment of the invention, dedicated additional implantsare made using the following mask levels: P-BL, P-POLYDOP, P-BODY,P-LDD, P-S/D, and P-CONT, where P-BL refers to a P-type doping of theburied layer, and P-POLYDOP refers to a mask level enabling P⁺ doping ofPolysilicon for the PFET devices. In this case an additional N-POLYDOPmask level is used for the N+ doping of polysilicon for NFET devices.

Thus, the complete mask set in the exemplary BiCMOS process, accordingto embodiments of the invention, includes a maximum of 18 to 20 levels.This process flow allows a design of all the exemplary electroniccomponents shown in FIGS. 9 through 19 which may be used to manufacturea power IC.

The process flow using the basic mask set needed to manufacture diodepower devices described herein is the same as discussed above for theBiCMOS technology. The process is based on an SOI substrate with a P⁻handle wafer, and an N⁻ active layer in the case of an Nch MOSFET. Thisprocess flow can include the following main steps when forming diodestructures disclosed herein:

Lateral dielectric isolation by etching a trench through the activelayer, and fill it with oxide or a combination of oxide and polysilicon(LTI Mask);

Deep implantation of boron to create a local deep P⁺ well with aconcentration peak close to the buried oxide interface (BL Mask);

Pattern a mask to define the position of the Gate trenches (TRGMask—optional);

Etch the gate trench with rounded bottom and top corners, grow a thermalGate oxide, and fill the trench with polysilicon (optional—for structurelike FIGS. 15-15B that include gate trenches only);

Dope the deposited polysilicon by phosphor implantation and anneal, anddeposit a silicide layer on the top;

Pattern the polycide layer (POLY Mask);

Implant boron to create a body region self-aligned to the edge of thepolycide layer for PN diodes, and self-aligned to the polycide layeropenings used to form a button body contact in Schottky diodes. Performbody diffusion with a dedicated thermal anneal;

Implant phosphor or arsenic to create a lightly doped drain extension(called lightly doped drain (LDD)) at the other edge of the polycidelayer (LDD Mask);

Create highly doped cathode regions by shallow arsenic implantation (S/DMask);

Deposit field oxide to assure an electric isolation of the gate stackstructure;

Etch a shallow source (anode) contact trench (CONT Mask) and implant BF₂through the trench bottom (plug implant) to assure a good ohmic contactto body and deep P⁺ regions.

Deposit and sinter a silicide film (e.g. Ti/TiN) lining the trenchcontact walls to create an electric short between anode, body and deepP⁺ regions;

Pattern the contact silicide layer (FPL Mask);

Deposit an interlayer dielectric film (ILD) and applychemical-mechanical polishing step (CMP) to achieve a planar topsurface;

Etch via openings to access anode, cathode, and gate contact areas (ViaMask). Fill vias with tungsten plugs (Ti/TiN/W) and apply CMP step toplanarize the top surface again.

Perform a two-step RTP anneal to stabilize the Schottky contact barrier.

Deposit and pattern thick Al layer to create top electrodes with anode,cathode, and gate bus structures (Metal Mask).

As discussed above this technology requires few mask levels. An optionalmask can be used to create an electrical contact to the substrate byetching a deep trench through the active layer and the buried oxide, andfill it with oxide and doped polysilicon.

Processing details are well known to those skilled in the art and willtherefore not be presented in further detail herein. By way of exampleonly and without limitation, illustrative values for certaintechnological process parameters are listed below for the case offabricating an exemplary 20-volt N-channel MOSFET:

-   -   SOI substrate: lightly doped handle wafer (e.g., <5e14 cm⁻³),        0.3-μm buried oxide, and 0.6-μm active film with a doping of        around 1e16 cm⁻³.    -   Buried P⁺ well: Boron implant with a dose of 2e13 cm⁻² and        energy of 180 keV.    -   Gate trench: 0.3 μm wide, 0.3 μm deep, and 0.3 μm long.    -   Polycide layer: 0.3-μm polysilicon and 0.1-μm WSi₂. Polycide        stripe width 0.45 μm covering gate trench, or 0.35 μm for the        case of the NFET without gate trenches    -   Body region: Boron implant with a dose of 3e13 cm⁻² and energy        of 30 keV, followed by a second boron implant with a dose of        4e13 cm⁻² and energy of 90 keV, and a 60 minutes anneal at 1000°        C.    -   LDD region: Phosphor implant with a dose of 6e12 cm⁻² and energy        of 60 keV.    -   S/D regions: Arsenic implant with a dose of 5e15 cm⁻² and energy        of 30 keV.    -   Contact trench: 0.4 μm wide and 0.25 μm deep.    -   Silicide film: Ti (300 Angstroms)/TiN (800 Angstroms) annealed        at 800° C.    -   Plug implant: BF₂ implant with a dose of 7e14 cm⁻² and energy of        30 keV.    -   Top metal: AlSiCu (1.5 μm thickness) patterned with 0.5 μm        metal-to-metal spacing.

The basic mask set needed to manufacture a NPN transistor as discussedabove can be used to form a power SOI BJT as described above inconnection with FIG. 12A-12D, or 12E as the case may be. The process isbased on an SOI substrate with a P− handle wafer, and an N-active layerand can include the following main steps:

Lateral dielectric isolation by etching a trench through the activelayer, and fill it with oxide or a combination of oxide and polysilicon(LTI Mask).

Deep implantation of boron to create a local deep P+ well with aconcentration peak close to the buried oxide interface (BL Mask).

Deposit and dope a polysilicon layer by phosphor implantation andanneal. Deposit a silicide layer on the top.

Pattern the polycide layer (POLY Mask).

Implant boron to create a base region self-aligned to the edge of thepolycide layer (BODY Mask). Perform base diffusion with a dedicatedthermal anneal (e.g., 1000° C. for 60 min) to drive the implant underthe whole length of the base/gate.

Implant phosphorous or arsenic to create a lightly doped collectorextension (similar to the LDD in the LDMOS structure) (LDD Mask).

Create highly doped emitter and collector regions by shallow arsenicimplantation (S/D Mask).

Etch a shallow button contact trench (CONT Mask) and implant BF₂ throughthe trench bottom (plug implant) to assure a good Ohmic contact to baseand deep P+ regions.

Deposit and sinter a silicide film (e.g. Ti/TiN) lining the trenchcontact walls.

Pattern the contact silicide layer allowing a lateral extension to asmall overlap of the polycide layer to create an electric contactbetween the deep P+ well and the polycide layer. As with the MOSFETprocess, this same mask can be used to define the optional field plateextension

Deposit an interlayer dielectric film (ILD) and apply chemo-mechanicalpolishing step (CMP) to achieve a planar top surface.

Etch via openings to access emitter, collector and base contact areas(VIA Mask). Fill vias with tungsten plugs (Ti/TiN/W) and apply CMP stepto planarize the top surface again.

Deposit and pattern thick Al layer to create top electrodes withemitter, collector, and base bus structures (METAL Mask).

As discussed above the processing of an NPN transistor requires 10 masklevels. An optional mask can be used to create an electrical contact tothe substrate by etching a deep trench through the active layer and theburied oxide, and fill it with oxide and doped Polysilicon.

In order to create a PNP BJT in the same process flow, a modified masksub-set has to be used. Dedicated, additional implants are made usingthe following mask levels: P-BL, P-POLYDOP, P-BODY, P-LDD, P-S/D, andP-CONT.

Both types of BJT transistors can be integrated within an SOI-BiCMOSprocess flow with maximum of 18 mask levels as discussed in thedisclosure on SOI-BiCMOS. This process flow allows a design of varietyof electric components which may be used to manufacture a power IC.

The processing details are well known to people skilled in the art.Values of the critical technological parameters are listed above for thecase of a 20V BiCMOS technology used as an example

In embodiments, the source and drain busses are placed at the oppositeends of the transistor active cells with a gate bus created by thepolycide layer running along the center of the layout. The source anddrain metal contacts have an interleaved finger structure, and theirpitch equals the pitch of one active cell as shown in, for example, 9A,10, 10A, 11 or 19. A predefined number of active cells is connectedtogether through the bus structure into a large macro-cell with lateraldimension of a few hundred microns (e.g. 300 by 300 μm). This macro-cellapproach enables a transistor layout scalable to a large area (e.g. 1 to5 mm²) by repetition and connection of the predefined macro-cells.Various techniques for forming macro-cells comprising a number ofindividual active cells (e.g., checkerboard layout) and repeatinggrouping those macro-cells together to function as an individual deviceare described in, for example, U.S. Pat. No. 7,446,375, issued Nov. 4,2008, the entirety of which is hereby incorporated by reference herein.However, unlike the '375 patent, which describes a device with verticalcurrent flow to a backside electrode, both source and drain terminalsand source and drain busses of the present LDMOS power deviceembodiments, which employ lateral current flow, would be formed on a topside of the semiconductor substrate. It should be understood that thismacro-cell approach is applicable to all power devices disclosed herein,including MOSFET and BJT transistors and diodes.

Features and advantages achieved according to embodiments of theinvention include, but are not limited to, one or more of the following,although a given embodiment may not necessarily include all of thesefeatures or only these features:

-   -   Exploits unique aspects of the BiCMOS process, like        manufacturing of all integrated power devices with the same set        of process steps;    -   Doping and placement of the deep buried well defines the        breakdown voltage and the location of avalanche impact        ionization within all SOI power devices; i.e., a clamping diode        is effectively integrated in the device, thereby assuring high        avalanche ruggedness;    -   BiCMOS process flow is defined with an aim to minimize SOI-LDMOS        power losses in SMPS applications. Other power devices like PN        diodes Schottky diodes, and BJTs are obtained by modification of        the SOI-LDMOS structure;    -   PN diode is obtained by removing N⁺ source region from N-channel        LDMOS structure;    -   Schottky diode is obtained by removing P body region from the PN        diode structure;    -   Bipolar transistor is obtained by removing the electrical short        between source and body regions. Gate stack is connected to the        body region and builds a current bus structure used as a base        terminal;    -   Chip scale package (CSP) or wafer level packaging (WLP) is        adopted to create current terminals on the top surface of the        finished die.

In the case of a wired package, the current bus stripes lead to terminalpad areas. If a chip-scale assembly (CSP or WLP) is adopted, which hasthe advantage of a smaller product footprint and less parasiticcomponents like package resistance and inductance, then the current busstructure 2308 (which corresponds to, for example, gate, drain andsource top electrodes 2032 (or other contacts in the case of diode orBJT embodiments) is contacted to a ball contact 2306 through vias 2302and a redistribution layer 2304, as shown schematically in FIG. 23.Returning to the earlier discussion regarding macro-cells, individualsource, drain and gate busses can be connected to multiple source, drainand gate terminals, of multiple like or identical devices, therebyallowing those multiple devices to operate as a single macro-celldevice. Multiple macro-cell devices can then be connected together tooperate as one power device by, for example, the redistribution layer2304. That is, an individual contact 2306 can be connected to multiplesource busses 2308, for example, and the like for the gate and draincontacts 2306.

As previously stated, an important benefit of embodiments of theinvention is the ability to easily facilitate the integration of powercircuits and/or components (e.g., drivers and power switches) on thesame silicon substrate as corresponding control circuitry forimplementing a power control device. By way of example only and withoutlimitation, FIGS. 21A through 22E are cross-sectional views depicting atleast a portion of an exemplary BiCMOS process flow for integrating twopower devices on the same substrate, according to an embodiment of theinvention. Specifically, FIGS. 21A through 22E conceptually illustratean exemplary process flow which utilizes the same process steps forintegrating a power N-channel MOSFET and a power Schottky diode on acommon SOI substrate. Other devices, such as, for example, PN diodes andBJTs, can be fabricated within the same process step sequence.

With reference to FIG. 21A, at least two active regions 2102 and 2104are shown. Each of the active regions 2102 and 2104 in which deviceswill be formed, in this embodiment, comprise respective N⁻ activeregions 2106 separated by a lateral isolation trench 2108, although theactive regions 2106 may be of a different conductivity type in otherembodiments. Lateral isolation trenches 2108 are used to separate otheradjacent active regions 2106 for forming other devices and/orstructures. Using process steps previously described, the active regions2106 are formed on a common buried oxide layer 2110 which in turn isformed on an N or P-type substrate 2112. Buried P⁺ wells 2114 are formedin the respective N⁻ active regions 2106, proximate an interface betweenthe buried oxide layer 2110 and the active regions.

In FIG. 21B, a gate oxide layer 2120 is formed over the surface of theSOI structure. A layer of polysilicon 2122 is deposited on the gateoxide layer 2120 and patterned to form gate structures. A silicide layer2124 is optionally deposited on the polysilicon gate structures 2122.Then, P body regions 2116 are formed by doping the active region 2106over at least a portion of the buried wells 2114, whereby the P implantused to form the P body regions is self-aligned to one edge of thepolycide regions. N regions 2118 are also formed in the active layer2106. In active region 2102, the N region 2118 is formed between the Pbody regions 2116 allocated to build a MOSFET structure (e.g., NMOSdevice 1000 shown in FIG. 10). The same implantation step is used toform N regions 2118 in the structure of a Schottky diode, as previouslyshown in FIG. 14B. FIG. 21C shows doped N+ regions 2126 formed in P bodyregions 2116 and N regions 2118. An oxide layer 2128 is formed over atleast a portion of the upper surface of the SOI structure.

With reference to FIG. 21D, trenches 2130 are formed substantiallyvertically through the oxide layer 2128, the P body region 2116, andcontacting the buried P+ well 2114. A silicide or titanium/titaniumnitride layer 2132 is formed on sidewalls and a bottom wall of thetrenches 2130. The silicide layer 2132 lining the trenches 2130 contactthe N⁺ doped regions 2126 in the P body region 2116. Shield field plates2134, which in this embodiment are formed as lateral extensions of thesilicide layers 2132 lining the trenches 2130, overlap the gatestructures and come into close proximity with an oxide interface alongthe N active region 2118. An oxide layer 2136 is then formed over atleast a portion of the upper surface of the SOI structure. FIG. 21Edepicts the oxide layer 2136 etched to form contact trenches (i.e.,vias), which are subsequently filled with a metal (e.g., aluminum), oran alternative conductive material, to form device contacts 2138.

FIGS. 24A and 24B are alternative cross-sectional views depicting atleast a portion of an example N-channel LDMOS transistor (“transistor”)2400 formed on a bulk semiconductor substrate 2401, according to anembodiment of the invention. As shown, the transistor 2400 includes: thebulk semiconductor substrate 2401; a buried well (i.e. “Deep Pt” or“Buried P⁺ Well”) 2402; an N-type region 2404; a source region 2405;gate trenches 2406; a drain region 2407; a lightly doped drain (LDD)extension region 2408; an oxide region 2409; a drain contact 2410; agate shield plate 2412; an avalanche breakdown region 2413; a gateregion 2414; a top surface 2415; a source contact 2416; a silicide layer2418, and a body region (i.e. “P Body”) 2420. As shown in FIG. 24B, thetransistor 2400 additionally includes gate oxide 2422 and a lateralisolation region 2424. The avalanche breakdown region 2413 is a regionof the buried well 2402 that is in closest proximity to the draincontact 2410. The avalanche breakdown region 2413 is labeled forillustration purposes only.

In some embodiments, the bulk semiconductor substrate 2401 is a P-typesubstrate (e.g. P, P⁺, or P⁻). In other embodiments, the bulksemiconductor substrate 2401 is an N type substrate (e.g. N, N⁺, N⁻). Instill other embodiments, other doping schemes are used, as will becomeapparent to those skilled in the art given the teachings herein. In someembodiments, the N-type region 2404 is a region of the active region andhas N⁻ background doping.

As was previously discussed with reference to FIG. 21A, illustrativeexamples of the active region 2102 and the active region 2104 of FIG.21A are shown formed on the buried oxide layer 2110. By contrast, theactive region of the transistor 2400 of FIG. 24A is formed directly onthe bulk semiconductor substrate 2401. In some embodiments, the activeregion of the transistor 2400 includes the buried well 2402, the N-typeregion 2404, the source region 2405, the drain region 2407, the LDDextension region 2408, and the body region 2420.

In some embodiments, the transistor 2400 is fabricated using a BiCMOSprocess technology on the bulk semiconductor substrate 2401. Thefabrication process of the transistor 2400 is the same or similar to thefabrication process described in detail with reference to FIG. 20Athrough FIG. 20F and FIG. 21A through FIG. 21E, contrasted by theexclusion of a buried oxide layer.

The gate trenches 2406 of the transistor 2400 include multiple trenches(i.e., trench stripes). Each trench has sidewalls and bottom walls linedwith the gate oxide 2422, or an alternative dielectric, and filled withpolysilicon material, or an alternative conductive material whichadditionally forms the gate region 2414. For an NFET device formedaccording to some embodiments, the polysilicon material is preferablydoped with phosphorous, with a doping concentration of greater thanabout 10¹⁹/cm³. For a PFET device, the polysilicon material ispreferably doped with boron having a doping concentration of about10¹⁹/cm³.

The top surface of the gate region 2414 is shown optionally covered bythe silicide layer 2418. In some embodiments, the silicide material(e.g., titanium silicide (TiSi) or tungsten silicide (WSi)) has lowresistivity. In some embodiments, the silicide material is depositedonto the polysilicon material of the gate region 2414 using a knownsilicide deposition process (e.g., chemical vapor deposition (CVD),sputter deposition, etc.). The silicide layer 2418, which forms apolycide electrode in the transistor 2400, reduces a gate resistance ofthe transistor 2400.

The gate trenches 2406 are preferably formed as a group of parallelstripes which, when properly biased, affect a current flow therebetween.In some embodiments, the trenches of the gate trench 2406 are formedunderneath the polycide electrode along a path of current flow in theactive layer of the transistor 2400. In this manner, the trenches of thegate trenches 2406 increase an effective gate width in the transistor2400, among other advantages. Each gate trench 2406 extendssubstantially vertically from the top of the active layer of thetransistor 2400 (or top of the body region 2420), through the bodyregion 2420, and at least partially into the buried well 2402. In someembodiments, each gate trench 2406 extends through the buried well 2402.The gate oxide 2422 lining the sidewalls and bottom walls of the gatetrenches 2406 prevents direct electrical connection between thepolysilicon material filling the gate trenches 2406 and the buried well2402.

The gate trenches 2406 are placed between the source contact 2416 andthe drain contact 2410 and are formed (e.g., etched) to be substantiallyparallel to a current path. The gate trenches 2406 can deplete orenhance a gate/body interface, controlling the current flow through aninversion channel formed in the transistor 2400. As a result, aconduction current flows in the body region 2420 between the gatetrenches 2406. Thus, the conduction current flows from the sourcecontact 2416, through the source region 2405, through the body region2420 between walls of the trench gates 2406, through the LDD extensionregion 2408, through the drain region 2407 and to the drain contact2410.

Another trench structure, formed deeper than the gate trenches 2406, isused to create the lateral isolation region 2424 between integratedcomponents. The lateral isolation region 2424, can be formed, forexample, by etching from the top surface 2415 of the transistor 2400 orfrom the top of the active layer of the transistor 2400, through theactive layer, to the bulk semiconductor substrate 2401. The lateralisolation region 2424 can be filled with oxide, or a combination ofoxide and polysilicon. An optional deep trench cut (i.e., etch), notexplicitly shown, to the bulk semiconductor substrate 2401 can be usedas a substrate contact. This optional trench is preferably filled withdoped polysilicon material, or an alternative conductive material, toensure good ohmic (i.e., low resistance) contact to the bulksemiconductor substrate 2401.

As was described, the silicide layer 2418 is formed on the gate region2414 (a polysilicon gate structure), thereby forming a polycide layer(also referred to as silicided polysilicon). The silicide layer 2418forms a gate bus leading to a gate terminal (G) located in a thirddimension (not explicitly shown, but implied). The silicide layer 2418is preferably formed using a known deposition process (e.g., CVD,sputtering, etc.). The source contact 2416 and the drain contact 2410are formed as metal-filled vias reaching a patterned top or overlayingmetal layer (not explicitly shown, but implied) and form the sourceterminal (S) and the drain terminal (D), respectively, of the transistor2400.

The transistor 2400 includes an integrated PN clamping diode. Theintegrated PN clamping diode is formed by the buried well 2402 and theLDD region 2408. The buried well 2402 is locally implanted at the bottomof the active region (below the body region 2420). This was illustratedin FIG. 20A, which shows a buried well 2004 implanted at the bottom ofthe active region 2002.

The clamping diode is as an integral part of each active cell (e.g. anactive region fully or partially isolated from other active regions bylateral insulating structures such as the lateral isolation structure2424). The clamping diode “pins” the avalanche breakdown away from thegate oxide 2422 and close to region of the buried well 2402 that isclosest to the drain contact 2410 within the volume of the active layer.The region of the buried well 2402 that is closest to the drain contact2410 is indicated as the avalanche breakdown region 2413, located at theupper right corner of the buried well 2402.

Properties or characteristics of the buried well 2402 (e.g., the dopinglevel, doping type and/or a location of the buried well 2402) areconfigured, determined or selected in a manner which results insubstantially clamping the breakdown voltage at the PN junction formedbetween the region of the buried well 2402 that is in closest proximityto the drain contact 2410, as was described earlier. By selecting orsetting the one or more characteristics of the buried well 2402, adistribution of an electric field in the transistor 2400 is affected. Byproviding an ability to clamp the avalanche at a desired location in thetransistor 2400, the transistor 2400 formed in accordance with one ormore aspects of the invention is well-suited for use in a powerswitching application. The clamping diode increases the avalancheruggedness of the transistor 2400 without causing any reliabilityissues. This transistor design minimizes the injection of hot carriersinto oxides, improving the long-term reliability of the transistor 2400.

Additionally, the transistor 2400 advantageously has a reducedgate-to-drain capacitance (C_(gd)) in comparison to standard LDMOSdevices, due at least in part to the effect of the gate shield plate2412. In the illustrated embodiment, the gate shield plate 2412, or analternative shielding structure, is formed as a lateral extension of aconductive layer lining the walls of a trench for the source contact2416. The gate shield plate 2412 overlaps the gate region 2414 (e.g.,polysilicon structure) and is proximate to an oxide interface along theLDD extension region 2408. In this illustrative embodiment, the trenchfor the source contact 2416 is formed on the left-hand side of thetransistor 2400, having side walls and a bottom wall lined with gateshield plate 2412 and is filled with top metal (e.g. of the sourcecontact 2416). In some embodiments, the conductive layer lining thewalls of the trench for the source contact 2416 is preferably depositedas a titanium (Ti)/titanium nitride (TiN) stack, but may be also formedof other materials, such as, for example, a titanium (Ti)/tungstensilicide (WSi) film.

The gate shield plate 2412 functions primarily as a field plate,distributing (e.g., stretching) an electric field along an interfacewith the top oxide region 2409 away from an edge (e.g., bottom rightcorner) of the gate region 2414 nearest the drain region 2407. The gateshield plate 2412 also functions as a shield that helps to reducegate-to-drain capacitance, C_(gd) (so called Miller capacitance, whichdetermines or affects the switching speed of the transistor 2400), at apositive bias of the drain region 2407 and further improves gate oxidereliability. The electric field peak appearing at the drain side cornerof the gate region 2414 is split between the gate corner and the end ofthe gate shield plate 2412, thereby reducing the electric field peakvalue and inhibiting early injection of hot carriers into the top oxideregion 2409. C_(gd) can also be reduced by depleting the LDD extensionregion 2408.

As has been described, in some embodiments, the active region is formeddirectly on a bulk semiconductor substrate. In other embodiments, theactive region is formed in an epitaxial layer formed on a bulksemiconductor substrate. An epitaxial layer is a crystalline layer grownor deposited on a crystalline substrate (e.g. the bulk semiconductorsubstrate).

Forming an epitaxial layer on the bulk semiconductor substrate canadvantageously allow use of a heavily-doped semiconductor substrate inorder to control the potential of the substrate, while maintaining alightly-doped surface layer for which to manufacture active devices.Additionally, a buried layer can be implanted into the semiconductorsubstrate before the epitaxial layer is formed, resulting in a buriedlayer below the epitaxial layer.

FIGS. 25A and 25B are alternative cross-sectional views depicting atleast a portion of an example N-channel LDMOS transistor 2500 formed inan epitaxial layer 2511 which is formed on a bulk semiconductorsubstrate 2501, according to an embodiment of the invention. As shown,the transistor 2500 includes: the bulk semiconductor substrate 2501; aburied well (i.e. “Deep Pt” or “Buried P+ Well”) 2502; an N-type region2504; a source region 2505; gate trenches 2506; a drain region 2507; aLDD extension region 2508; an oxide region 2509; a drain contact 2510;the epitaxial layer 2511; a gate shield plate 2512; an avalanchebreakdown region 2513; a gate region 2514; a top surface 2415; a sourcecontact 2516; a silicide layer 2518, and a body region (i.e. “P Body”)2520. As shown in FIG. 25B, the transistor 2500 additionally includesgate oxide 2522 and a lateral isolation region 2524. These depictedportions of the N-channel LDMOS transistor 2500 have similar purpose,function, and attributes of described portions of the N-channel LDMOStransistor 2400 described previously with respect to FIG. 24A throughFIG. 24B. However, unlike the transistor 2400, the transistor 2500includes the epitaxial layer 2511.

In some embodiments, the active region of the LDMOS transistor 2500 isformed in the epitaxial layer 2511 that is formed on the bulksemiconductor substrate 2501. In some embodiments, the bulksemiconductor substrate 2501 is a P-type substrate (e.g. P, P⁺, or P⁻).In other embodiments, the bulk semiconductor substrate 2501 is an N typesubstrate (e.g. N, N⁺, N⁻). In still other embodiments, other dopingschemes are used, as will become apparent to those skilled in the artgiven the teachings herein. Although the epitaxial layer 2511 is shownas a separate layer, in some embodiments all or a portion of the activeregion of the LDMOS transistor 2500 is formed in the epitaxial layer2511. That is, in some embodiments, the active region of the LDMOStransistor 2500 is formed in the epitaxial layer 2511 and is not formedon the epitaxial layer 2511. However, in other embodiments, the activeregion of the LDMOS transistor 2500 is formed on the epitaxial layer2511 and is not formed in the epitaxial layer 2511. In still otherembodiments, the active region of the LDMOS transistor 2500 is formed inand on the epitaxial layer 2511.

The epitaxial layer 2511 can be an N or P type epitaxial layer (N+/− orP+/− Epi-Layer) that is formed on the N or P type bulk semiconductorsubstrate 2501. In some embodiments, the active region includes theburied well 2502, the N-type region 2504, the source region 2505, thedrain region 2507, the LDD extension region 2508, and the body region2520.

The bulk semiconductor substrate 2501 can be un-doped, or have a dopinglevel that ranges from lightly doped to highly doped. In someembodiments, the epitaxial layer 2511 is a P or P⁻ epitaxial layer on aP or P⁺ bulk semiconductor substrate (2501). In other embodiments, theepitaxial layer 2511 is an N or N⁻ epitaxial layer on a P or P⁺ bulksemiconductor substrate (2501). The epitaxial layer 2511 can beun-doped, or have a doping level that ranges from lightly doped tohighly doped. For example, a lightly doped N⁻ material can be used asthe epitaxial layer 2511 to form the N-type region 2504. A region of theN-type region 2504 can then be counter-doped to form the buried well2502.

FIG. 26 is a cross-sectional view depicting at least a portion of anexample N-channel LDMOS transistor (“transistor”) 2600 formed on a bulksemiconductor substrate 2601, according to an embodiment of theinvention. As shown, the transistor 2600 includes: the bulksemiconductor substrate 2601; a buried well (i.e. “Deep Pt”) 2602; anN-type region 2604; a source region 2605; a drain region 2607; a LDDextension region 2608; an oxide region 2609; a drain contact 2610; agate shield plate 2612; an avalanche breakdown region 2613; a gateregion 2614; a top surface 2615; a source contact 2616; a silicide layer2618, and a body region (i.e. “P Body”) 2620. The avalanche breakdownregion 2613 is a region of the buried well 2602 that is in closestproximity to the drain contact 2610. The avalanche breakdown region 2613is labeled for illustration purposes only.

The LDMOS transistor 2600 is designed as a simplification of thetransistor 2400 shown in FIGS. 24A and 24B. As apparent from FIG. 26,one simplification in the fabrication of the transistor 2600, comparedto the transistor 2400 shown in FIGS. 24A and 24B, includes removal ofthe gate trenches (2406 in FIGS. 24A and 24B). A primary impact on theperformance of the LDMOS transistor 2600 is a smaller gate width perunit area, which increases on-resistance, R_(ON), of the resultingdevice. This can be leveraged by making the channel length shorter, asthe alignment restriction related to an overlap of the gate polysiliconover gate trench endings is removed. Other features and characteristicsof the MOSFET transistor 2600 remain essentially the same as for LDMOStransistor 2400.

Similarly, FIG. 27 is a cross-sectional view depicting at least aportion of an example N-channel LDMOS transistor (“transistor”) 2700formed in and/or on an epitaxial layer 2711 which is formed on a bulksemiconductor substrate 2701, according to another embodiment of theinvention. As shown, the transistor 2700 includes: the bulksemiconductor substrate 2701; a buried well (i.e. “Deep Pt”) 2702; anN-type region 2704; a source region 2705; a drain region 2707; a LDDextension region 2708; an oxide region 2709; a drain contact 2710; theepitaxial layer 2711; a gate shield plate 2712; an avalanche breakdownregion 2713; a gate region 2714; a top surface 2715; a source contact2716; a silicide layer 2718, and a body region (i.e. “P Body”) 2720.

The transistor 2700 is designed as a simplification of the LDMOStransistor 2500 shown in FIGS. 25A and 25B, the simplification includingthe removal of the gate trenches (2506 in FIGS. 25A and 25B). Otherfeatures and characteristics of the LDMOS transistor 2700 remainessentially the same as for the LDMOS transistor 2500.

The fabrication process of the transistor 2400, the transistor 2500, thetransistor 2600 and/or the transistor 2700 is the same or similar to thefabrication process described in detail with reference to FIG. 20Athrough FIG. 20F and FIG. 21A through FIG. 21E, contrasted by theexclusion of a buried oxide layer. Although the epitaxial layer 2711 isshown as a separate layer, in some embodiments all or a portion of theactive region of the LDMOS transistor 2700 is formed in the epitaxiallayer 2711.

In some embodiments, MOSFET power devices include MOSFET powertransistors, such as an N-channel LDMOS transistor, as described herein.In some embodiments, the active region formed on a bulk semiconductorsubstrate includes MOSFET power transistors and integrated controlcircuitry. In some embodiments, an active region formed in and/or on anepitaxial layer formed on a bulk semiconductor substrate includes MOSFETpower transistors and integrated control circuitry.

At least a portion of the embodiments of the invention may beimplemented in an integrated circuit. In forming integrated circuits,identical die are typically fabricated in a repeated pattern on asurface of a semiconductor wafer. Each die includes at least one devicedescribed herein, and may include other structures and/or circuits. Theindividual die are cut or diced from the wafer, then packaged as anintegrated circuit. One skilled in the art would know how to dice wafersand package die to produce integrated circuits. Integrated circuits somanufactured are considered part of this invention.

An integrated circuit in accordance with embodiments of the inventioncan be employed in essentially any application and/or electronic systemin which power management techniques may be employed. Suitableapplications and systems for implementing techniques according toembodiments of the invention may include, but are not limited to,portable devices, including smart phones, laptop and tablet computingdevices, netbooks, etc. Systems incorporating such integrated circuitsare considered part of embodiments of the invention. Given the teachingsof embodiments of the invention provided herein, one of ordinary skillin the art will be able to contemplate other implementations andapplications of the techniques of embodiments of the invention.

The illustrations of embodiments of the invention described herein areintended to provide a general understanding of the structure of variousembodiments, and they are not intended to serve as a completedescription of all the elements and features of apparatus and systemsthat might make use of the structures described herein. Many otherembodiments will become apparent to those skilled in the art given theteachings herein; other embodiments are utilized and derived therefrom,such that structural and logical substitutions and changes can be madewithout departing from the scope of this disclosure. The drawings arealso merely representational and are not drawn to scale. Accordingly,the specification and drawings are to be regarded in an illustrativerather than a restrictive sense.

Embodiments of the inventive subject matter are referred to herein,individually and/or collectively, by the term “embodiment” merely forconvenience and without intending to limit the scope of this applicationto any single embodiment or inventive concept if more than one is, infact, shown. Thus, although specific embodiments have been illustratedand described herein, it should be understood that an arrangementachieving the same purpose can be substituted for the specificembodiment(s) shown; that is, this disclosure is intended to cover anyand all adaptations or variations of various embodiments. Combinationsof the above embodiments, and other embodiments not specificallydescribed herein, will become apparent to those of skill in the artgiven the teachings herein.

The abstract is provided to comply with 37 C.F.R. § 1.72(b), whichrequires an abstract that will allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. In addition, in the foregoing DetailedDescription of Preferred Embodiments, it can be seen that variousfeatures are grouped together in a single embodiment for the purpose ofstreamlining the disclosure. This method of disclosure is not to beinterpreted as reflecting an intention that the claimed embodimentsrequire more features than are expressly recited in each claim.

Given the teachings of embodiments of the invention provided herein, oneof ordinary skill in the art will be able to contemplate otherimplementations and applications of the techniques of embodiments of theinvention. Although illustrative embodiments of the invention have beendescribed herein with reference to the accompanying drawings, it is tobe understood that embodiments of the invention are not limited to thoseprecise embodiments, and that various other changes and modificationsare made therein by one skilled in the art without departing from thescope of the invention.

What is claimed is:
 1. A metal-oxide-semiconductor field-effecttransistor (MOSFET) power device comprising: a bulk semiconductorsubstrate; an active region formed on the bulk semiconductor substrate;a buried well having a first conductivity type formed in the activeregion on the bulk semiconductor substrate; a source region having asecond conductivity type formed in the active region proximate an uppersurface of the active region, the source region being electricallyconnected to the buried well; a drain region having the secondconductivity type formed in the active region proximate the uppersurface of the active region and spaced laterally from the sourceregion; a body region having the first conductivity type formed in theactive region between the source region and the drain region on at leasta portion of the buried well; a gate formed above the active regionproximate the upper surface of the active region, wherein the gate iselectrically isolated from the active region by a gate insulating layer;a drain terminal formed on the upper surface of the active region andelectrically connected to the drain region; a source terminalelectrically connected to the source region; a gate terminalelectrically connected to the gate; and a shielding structure formedproximate the upper surface of the active region between the gate andthe drain region, wherein the shielding structure comprises a fieldplate configured to control an electric field distribution along a topoxide interface away from an edge of the gate nearest the drainterminal; wherein the buried well has a first end below the sourceterminal and a second end that extends partially below the drain region,the second end being laterally spaced from the drain terminal betweenthe drain terminal and the body region; and wherein the buried well isconfigured, in conjunction with the drain region, to form a clampingdiode operative to position a breakdown avalanche region between theburied well and the drain terminal, and a breakdown voltage of theMOSFET power device is a function of one or more characteristics of theburied well.
 2. The MOSFET power device of claim 1, wherein: theshielding structure overlaps at least a portion of the gate and isformed as an extension of a conductive layer contacting the sourceterminal.
 3. The MOSFET power device of claim 1, wherein: the fieldplate comprises a stepped extension of the shielding structure formedproximate the drain terminal.
 4. The MOSFET power device of claim 1,further comprising: an electrical connection between the source regionand the body region; wherein the electrical connection is a silicidelayer lining a source contact trench; and wherein the source contacttrench extends through the source region into the body region.
 5. TheMOSFET power device of claim 1, further comprising: control circuitryintegrated with the active region on the bulk semiconductor substrate,the control circuitry being configured to selectively control anoperation of the MOSFET power device.
 6. The MOSFET power device ofclaim 1, further comprising: a plurality of gate trench structuresextending vertically from the upper surface of the active region intothe buried well; wherein each of the gate trench structures includessidewalls and a bottom wall having an insulating material formedthereon; wherein each of the gate trench structures is filled with aconductive material; wherein the gate trench structures are electricallyconnected to the gate; wherein a voltage applied to the gate isoperative to modulate a conduction current which flows between the gatetrench structures through the active region; and wherein an amplitude ofthe conduction current is controlled as a function of the voltage.
 7. Ametal-oxide-semiconductor field-effect transistor (MOSFET) power devicecomprising: a bulk semiconductor substrate; an epitaxial layer formed onthe bulk semiconductor substrate; an active region formed in theepitaxial layer; a buried well having a first conductivity type formedin the active region in the epitaxial layer; a source region having asecond conductivity type formed in the active region proximate an uppersurface of the active region, the source region being electricallyconnected to the buried well; a drain region having the secondconductivity type formed in the active region proximate the uppersurface of the active region and spaced laterally from the sourceregion; a body region having the first conductivity type formed in theactive region between the source region and the drain region on at leasta portion of the buried well; a gate formed above the active regionproximate the upper surface of the active region, wherein the gate iselectrically isolated from the active region by a gate insulating layer;a drain terminal formed on the upper surface of the active region andelectrically connected to the drain region; a source terminalelectrically connected to the source region; a gate terminalelectrically connected to the gate; and a shielding structure formedproximate the upper surface of the active region between the gate andthe drain region, wherein the shielding structure comprises a fieldplate configured to control an electric field distribution along a topoxide interface away from an edge of the gate nearest the drainterminal; wherein the buried well has a first end below the sourceterminal and a second end that extends partially below the drain region,the second end being laterally spaced from the drain terminal betweenthe drain terminal and the body region; and wherein the buried well isconfigured, in conjunction with the drain region, to form a clampingdiode operative to position a breakdown avalanche region between theburied well and the drain terminal, and a breakdown voltage of theMOSFET power device is a function of one or more characteristics of theburied well.
 8. The MOSFET power device of claim 7, wherein: the bulksemiconductor substrate and the epitaxial layer are of the firstconductivity type; the bulk semiconductor substrate has a first dopinglevel; and the epitaxial layer has a second doping level.
 9. The MOSFETpower device of claim 7, wherein: the bulk semiconductor substrate is ofthe first conductivity type; the epitaxial layer is of the secondconductivity type; the bulk semiconductor substrate has a first dopinglevel; and the epitaxial layer has a second doping level.
 10. The MOSFETpower device of claim 7, wherein: the shielding structure overlaps atleast a portion of the gate and is formed as an extension of aconductive layer contacting the source terminal.
 11. The MOSFET powerdevice of claim 7, wherein: the field plate comprises a steppedextension of the shielding structure formed proximate the drainterminal.
 12. The MOSFET power device of claim 7, further comprising: anelectrical connection between the source region and the body region;wherein the electrical connection is a silicide layer lining a sourcecontact trench; and wherein the source contact trench extends throughthe source region into the body region.
 13. The MOSFET power device ofclaim 7, further comprising: control circuitry integrated with theactive region on the bulk semiconductor substrate, the control circuitrybeing configured to selectively control an operation of the MOSFET powerdevice.
 14. The MOSFET power device of claim 7, further comprising: aplurality of gate trench structures extending vertically from the uppersurface of the active region into the buried well; wherein each of thegate trench structures includes sidewalls and a bottom wall having aninsulating material formed thereon; wherein each of the gate trenchstructures is filled with a conductive material; wherein the gate trenchstructures are electrically connected to the gate; wherein a voltageapplied to the gate is operative to modulate a conduction current whichflows between the gate trench structures through the active region; andwherein an amplitude of the conduction current is controlled as afunction of the voltage.
 15. A metal-oxide-semiconductor field-effecttransistor (MOSFET) power device comprising: a bulk semiconductorsubstrate; an epitaxial layer formed on the bulk semiconductorsubstrate; an active region formed in and on the epitaxial layer; aburied well having a first conductivity type formed in the epitaxiallayer; a source region having a second conductivity type formed in theactive region proximate an upper surface of the active region, thesource region being electrically connected to the buried well; a drainregion having the second conductivity type formed in the active regionproximate the upper surface of the active region and spaced laterallyfrom the source region; a body region having the first conductivity typeformed in the active region between the source region and the drainregion on at least a portion of the buried well; a gate formed above theactive region proximate the upper surface of the active region, whereinthe gate is electrically isolated from the active region by a gateinsulating layer; a drain terminal formed on the upper surface of theactive region and electrically connected to the drain region; a sourceterminal electrically connected to the source region; a gate terminalelectrically connected to the gate; and a shielding structure formedproximate the upper surface of the active region between the gate andthe drain region, wherein the shielding structure comprises a fieldplate configured to control an electric field distribution along a topoxide interface away from an edge of the gate nearest the drainterminal; wherein the buried well has a first end below the sourceterminal and a second end that extends partially below the drain region,the second end being laterally spaced from the drain terminal betweenthe drain terminal and the body region; and wherein the buried well isconfigured, in conjunction with the drain region, to form a clampingdiode operative to position a breakdown avalanche region between theburied well and the drain terminal, and a breakdown voltage of theMOSFET power device is a function of one or more characteristics of theburied well.
 16. The MOSFET power device of claim 15, wherein: theshielding structure overlaps at least a portion of the gate and isformed as an extension of a conductive layer contacting the sourceterminal.
 17. The MOSFET power device of claim 15, wherein: the fieldplate comprises a stepped extension of the shielding structure formedproximate the drain terminal.
 18. The MOSFET power device of claim 15,further comprising: an electrical connection between the source regionand the body region; wherein the electrical connection is a silicidelayer lining a source contact trench; and wherein the source contacttrench extends through the source region into the body region.
 19. TheMOSFET power device of claim 15, further comprising: control circuitryintegrated with the active region on the bulk semiconductor substrate,the control circuitry being configured to selectively control anoperation of the MOSFET power device.
 20. The MOSFET power device ofclaim 15, further comprising: a plurality of gate trench structuresextending vertically from the upper surface of the active region intothe buried well; wherein each of the gate trench structures includessidewalls and a bottom wall having an insulating material formedthereon; wherein each of the gate trench structures is filled with aconductive material; wherein the gate trench structures are electricallyconnected to the gate; wherein a voltage applied to the gate isoperative to modulate a conduction current which flows between the gatetrench structures through the active region; and wherein an amplitude ofthe conduction current is controlled as a function of the voltage.